It may not be a legal excuse, but many people take it as a moral one.
I want to watch a movie that was released 3 years ago. I happened to see it was available for rental on iTunes last year, but for some obscure reason today it isn't. I can buy it for three times the price, but not rent it. Possibly because they have a deal with a TV station that will air it later this year, I don't know, I can only guess.
Anyway, I want to rent the film today, I'm fully willing to part with the fair price for it, but the content providers have decided they don't want the money, unless I pay a lot more to buy it outright.
Meanwhile, I have this app that can download any movie for free, illegally but with practically zero chance of getting caught. (In my country it's not even 100% clear whether downloading is illegal or not in this case, only uploading).
Guess what, content providers, you just lost the rental fee I was perfectly willing to pay but you refused to accept. And I don't even feel bad about it, quite the contrary, I feel entitled. Sure, it won't stand up in court, but it won't have to because nobody knows I did it.
Other example: if I buy a legal DVD with a movie for my children, they need to watch about a minute or so of copyright notices, "don't be a pirate", "you wouldn't steal a car", or similar bullshit before the movie can finally start. The illegally downloaded version starts right away. Guess which one we prefer. Isn't that ridiculous? They are degrading their legal customers' experience with a message that has zero effect on pirates! Clearly a lose-lose situation.