There's a difference between saying "I don't know" (even if he did) and asking for a review of it, he is perfectly allowed to do so. Batsmen don't have to walk and Hughes who quite clearly reacted as "Oh, it's dropped" then quickly threw the ball up in the air and claimed the wicket.
That said, I really couldn't care less, it's up to the umpires to give players out. I don't have an issue with either Hughes or Bell. Hughes isn't the first to claim a catch that wasn't, and given the situation, he wouldn't be the last. Bell isn't the first to use the UDRS as a tactical ploy rather than a definitive, "I didn't hit that" and review it within seconds of the finger going up.
Good ton from Prior, nice pace as well.