They actually can do whatever they want to. It's in the terms that you "signed" and hit agree to every time they have a new patch. You don't even own your own character. That's why selling it is against their terms of service because you're selling their property.
I know that, but in brazil we have a book named "consumer protection code" which is the major law for customers and supliers relationships, it cancel anything they said or even you signed if it is not a "fair" trade (the code has alot of stuff to say if it was a fair trade or not), before we could not do anything because WoW was not sold in brazil, now it is (we have wow in portuguese now too) so it suffers from the brazilian law, and here blizz would have to explain that kind of decision (or make you prove why not).