if it looks, sounds and smells too good to be true.. then it always is!
this was complete douchebaggery on behalf of CEX.. you should have stated the 40Ghs limit upfront, not 2 days later when you "remembered" that you don't want to go above 51% and also if you do not want to go above 51% you should have limited the registrations instead of changing the rules of the game midgame!
A lot of people have leasing contracts for rigs because of the promotion, really nice way of screwing your new users!