Going by the 3.5 patch notes, it would have taken a really exceptional leap of faith to believe gaining champion XP within the Arena might not be intended:
That’s the part that really irks me. Don’t get me wrong, I absolutely love the way we are no longer locked into playing one class only for an extended period of time. But please, devs, PLEASE learn to communicate with your community. You knew exactly that removing champion XP from Arena wouldn’t be hailed as a Good Thing. Impacts braced: none.
This would have gone down so much better if somebody had had the courage to speak up a month ago. Saying something like “look, we’d like to remove the class change fee, unfortunately the redesign requires us to cut champion XP from Arena, please discuss”. As it stands, this feels like yet another “bummer, they caught on quickly again, whose turn is it this time to get out from under the table and claim it was a bug all along?” situation. So much opportunity wasted, a man could weep.