Plus the heroes are always always young adults (or whatever age the book is aimed at) and there is very, very rarely explicit swearing or sexual content.

I generally don't read YA fiction anymore because I think the vast majority of it is shallow, cliched and boring, but there are some standouts that are original, subtle and intriguing. But there's nothing about those books that an adult couldn't enjoy. Perhaps they are written a little simpler, with heroes more easily relatable to young readers. But yeah, primarily a marketing tool.