Well you could say people did "buy" their beta. Blizzard gave beta access to anyone who purchased the Annual Pass, so I'm sure anyone willing to dish out $50 bucks or whatever for a beta key just got the annual pass.
MoP isn't getting the usual hype, and we all know why, but I don't think it will matter it will likely do as good as any other expansion. But I just don't think there's a demand for beta access as there was for Diablo 3.
Grats! I am taking a break from WoW until MoP (Mainly because of lack of content and D3) but would love to get a beta invite myself. Really looking foward to the monk, scenarios, and OH YEAH PET BATTLES NOT EVEN SARCASTIC. CAN'T WAIT!