I'll take a stab and answering your question -- both the one you literally asked and what I think is the spirit behind your post and question. (Sorry that this is at times redundant with other posts, it took me quite a while to write this, and those posts didn't exist when I started.)
You asked, "Why is making it easy for people to steal ethical?" The question itself makes it sound like you think it is not ethical to do so under any circumstances. But this leads us to absurdities. Is making it easy for child pornographers to send pictures to each other unethical? Yes? Ok, so adding "file transfer" capabilities to instant messaging clients must be unethical. Is making it easy for people to counterfeit goods unethical? Yes? Ok, so running eBay must be unethical. That's the same logic as saying "TPB makes it easy for people to steal, therefore running TPB must be unethical." At this point, I'm going to assume we agree that facilitating an unethical activity is not necessarily unethical.
On the other hand, I do agree that facilitating an unethical activity can be enough to make something unethical. You might argue that TPB was created with the intent to facilitate these activities, whereas the other examples I gave clearly were not, and that makes all the difference. To a large extent (but not 100%), I would agree with you on that point, and I will agree that it does make TPB's situation more grey. However, I don't think the case is closed on TPB at this point in the discussion -- if TPB's place on the ethical/unethical spectrum is based on the activities it facilitates, then we must ask how unethical those activities are.
I would argue that downloading software (or other types of bits) via torrent is never by itself unethical. Here is an example to illustrate why (this example happens to be true). Just a few months ago, I reinstalled Windows. There was a shareware application for which I had bought a full license, and I wanted to reinstall it. However, I couldn't find the install file. I still had my serial number in an email, but the version of the software available for download on the website was already at the next major version number, and so my serial number wouldn't work. Perhaps I could have contacted the company and asked for them to make an older version of the application available, but there was no guarantee that would have gotten me anything but wasted time. Instead, I found a torrent for the version I had bought (which included a keygen that I didn't need) and downloaded it that way. I think you will agree that there was nothing unethical about that, because I had already paid the author for the software.
Furthermore, even using software without paying for it may not be unethical. Here is another true example, actually involving the same piece of software. When I was in college, I used that application illegally -- that is, I think I found a serial number online which I hadn't purchased, and used that to eliminate the nag screen that would periodically interrupt use of the application (maybe I downloaded a crack, I don't recall). If that serial number or crack had not been available to me, I still would not have bought the full version -- it just wasn't worth it to me, relative to the amount of money I had. So, when we compare the two possible situations (one where there was a serial/crack available to me, and the hypothetical one where there was not), the author made the same amount of money from me either way ($0), but without the serial/crack I would have merely been less productive (I probably wouldn't have continued using the app). I benefited, and no one lost anything. Therefore, I do not consider what I did unethical. As a side note, it is entirely possible the author actually benefited... when I graduated and scored a fulltime telecommute job, I found myself using that application throughout the day while earning a healthy income, so I decided to go ahead and buy the software. Had I not been using it for years, I likely wouldn't have bought it then (I would possibly have found something else, or maybe I would have just been used to making due without it). In addition, while I was a grad student I was a teaching assistant, and often used that application on my laptop while teaching -- a student once asked me what it was I was using, and so I wound up telling the whole class what a wonderful tool it was. That definitely wouldn't have happened if I hadn't had the serial/crack. The point here is that unauthorized use isn't even itself a problem ethically -- it is merely the lost income to the author (when income is truly and realistically lost) which might cause it to be unethical.
I say "might" because even here, when looking at the big picture, there are mitigating factors when one consideres "benefit to society". Let's say you need to get paid at least $X in order for it to be worth your while to write this software as opposed to doing something else. I'm assuming your software provides some good to society, regardless of whether that good is more like "entertainment" or more like "enhanced productivity". You getting paid at least $X is good, because then you provide good to society through more (or improved) software -- but more people using your software rather than less is good too. Considering only these factors (and I know it's oversimplifying a lot here), the best thing for society is a situation where you are making at least $X from your software development efforts and as many people as possible have access to your software. It does not matter if all of those people have contributed to your $X or not. In evaluating whether the torrent of the software you wrote is really helping or hurting society, we would need to weigh the reduced income to you versus the benefit to society derived from the extra availability of your software. If it reduces your income to the point that you quit developing software, that's probably bad. But if it doesn't, it is entirely possible that the illegal distribution of your software has provided more good to society than harm to society.
The possibility of the situation I just described is a result of the fact that you are trying to work against the current state of the world: you have tied your business model to artifical scarcity. The resources to distribute your software to everyone who wants a copy are cheap, practically free even -- yet instead of embracing that (for example, maybe you could officially use P2P to distribute software, which has the potential to lower distribution costs), many companies fight it to the point of spending additional money and resources on DRM and various "copyright protection" schemes. There are ways to make your money without attempting to artificially restrict distribution, but probably not without changing your business model somewhat. The more companies change their business models to embrace the way things are rather than fight a losing battle against it, the better off society will be -- it is generally a more efficient way to do business to embrace reality rather than to waste resources fighting it, and in this case society can reap additional benefits as well (more people will have access to the software). To give a few examples, some lucky software developers are paid to work on open source projects -- they still get paid, and yet their work is freely distributed. Shareware actively encourages distribution. Some companies provide free downloads, but state that the program may only be used without a paid license by non-commercial entities; granted, they're still relying on laws to make their business model work, but at least they aren't worried about trying to artificially restrict distribution of their software. There are probably other ways to embrace things rather than fight them as well.
To summarize and bring this back to the point, TPB and other sites that facilitate P2P are providing a service which facilitates an action that by itself causes no harm. The only way it can cause harm is indirectly by causing someone not to pay you for software when they otherwise would have -- but if you are relying on artificial scarcity as an incentive for people to pay you money, then not only are you wasting time, effort, and/or emotional strain fighting a situation you could be embracing, that artificial scarcity itself is worse for society than the natural abundance which is the alternative.
I don't expect you to agree with me after reading this, but I hope you can at least see where I'm coming from.