Well, because they'd understand about Horcruxes, which are the main thing you have to take from it. Plus, the Unbreakable Vow. There'd be a couple jumps they'd have to take--basically, they'd have to accept Dumbledore as way smarter than he actually is, to know where the locket was located--but they'd get the basics and be able to follow the storyline. There's nothing important that is unexplained.

And oh my God, yes, I agree about PoA. Agh.