I hate when people do this, what if I download the app and it will be garbage ? Id lose money, that's also a reason for pirating content, when I search for a book on a certain topic I usually would discard 3 or 4 matches before choosing what I need, and the only way to discard these matches would be to read part of the book, the part which usually doesn't get in the previews.
Piracy is not just a way of getting content for free it's also a way not to lose money.
Low quality content is being intentionally sold to you. Just look at some books on programming, they trick you on thinking that they do have quality info, but in fact are just lazy rewrites of specifications or man pages. I also remember reading from HN about a guy who made millions by selling total garbage books on amazon.
Eventually if I found a good book I buy it as a way to thank the author, but buying books before reading them it is just a way to motivate people to produce more mediocre content.