emule has a rating system where any user can flag the content as good or bad, and I haven't gotten any fake content on it yet.
it can be slow...but it also encourages sharing and is great for obscure content!
maybe try considering it as an alternative.
and to answer your questions:
1) It's the RIAA, labels, and companies they hire to do it. Since it's intentional they try to do the best job they can to spread crap
2) People are just lame when it comes to that. In combination with #1, people just don't care or don't know how to delete the junk files (believe it or not)