I think he does it, because its easier for people to grasp and understand, and they won't get confused.
Think about it, Batman with a welsh accent, even if its not in the movie, if he is promoting the film, it might turn people off.
He's a smart guy, he doesn't stay in character per say like Pee Wee Herman, but he wants to sound like him and keep it close to the character.
Thats a pretty darn good work ethic and consideration if you ask me, thats going above and beyond the call for a film.
I see your point about why he would keep his accent. He does seem to have a very good work ethic. In interviews, Bale seems more concerned about his craft and his character than himself, unlike Cruise and other ego-maniacal Hollywood stars.