The mainstream computer companies are constantly banging on about how their products are easier to use, more user friendly, etc. than the competition. This is because the majority of the market (or the profitable bit) are essentially new computer users. Or are getting a computer of their own for the first time for recreational use, when they had generally only used computers in work/school for work/school stuff.
If the market was made up of experienced users, things wouldn't be sold as "easy to use", where easy to use is "possibly intuitive for those who don't get computer concepts, and will not RTFM". Cars (yeah, ignore the .sig) aren't sold as easy to use because the market for cars is made up of people who have to demonstrate they can work a car, and hence understand what features a car will and won't have, where they will probably be, and which ones to use and when. Car buyers don't need the steering wheel to be huge and green, they don't need a wizard to wind the windows down.
It wouldn't surprise me to find out that, world wide, every year there are more new users to the internet than the previous year. This means there's lots of new users who don't know how spam works, who don't know how affiliates work, who don't know how banner ads work, who don't know how Google works, who don't know about shit like those text advert links inserted into articles, etc.. These new users possibly will not think that v14gr4 is purposefully written like that, and might even think something like computer messages can suffer interference like a radio signal can.
New users also aren't used to the fonts. Slashdotters can no doubt tell the difference between l 1 I | and o O 0, and can identify the characters correctly when they aren't alongside the ones they could be confused with, but new users? No chance. v14gra might not look that odd to a new user, and so they don't spot it as suspect. You also can't buy viagra off the shelf (or at least, I don't think you can), so when presented with it human interest does kick in for some individuals.
I do sound like I am blaming new users, but I have been using the internet long enough to have seen new users come to the internet and wise up many times. Sometimes they barely wise up, sometimes they wise up very quick, but generally they stand to be manipulated the most when new to the web. There are people though who know they are limited, and so take things extra cautiously, though they are a fucking rare breed.
New users aren't necessarily used to the concepts that computers can produce copies for virtually no effort. Whilst there is a very clear cost to spam put through your front door, there isn't with email or other forms of spam, so even if the person has thought about the economics of advertising IRL, they might not get it right when it comes to computers. And so the spam could seem more legitimate than it is (not that I see any adverts as legitimate - they exist to manipulate your decision making processes, and I do not want that done to me. So I reject all advertising, everywhere).
Anyway, the answer? Education. And proper education, not asking MS what people should be learning. And not mandated computer science for all (though it should be available, if people want it). I dunno exactly what people need, but IME if some people had some basic knowledge of concepts like files and directories, programs and data, they would find using computers much less frustrating. I feel many proprietary products (and free ones that have copied paradigms) purposefully obscure what is going on so that the user becomes dependent on the proprietary product to do a job. The user can't learn what is going on, and if they did, they might change to a different product to do the same job. And that's bad for business.