Why not prewash? Does it give any clues as to why it would say that? Maybe they're afraid it'll shrink and fray so much you'll be left with something the size of a charm square!
I like Joann's and buy lots of fabric there, but I avoid their fat quarters. It seems like they have the LOWEST quality fabric in those. Some are OK, but most of them are really see-through. If you do buy them, make sure you open them up and check the weave.