Actually, it made a lot of sense to change. The premise was that if you want to be remembered, you have to check the box one time and you're done. But, if you didn't want to be remembered, you used to have to uncheck the box every time.
I suppose that rationale makes sense if some vB features didn't "break" when someone takes a long time completing an action. It's perceived as a timeout and the unremembered user is effectively "logged out." Lengthy posts, high scores in an arcade, all get lost. Very frustrating, especially for a newbie user who wouldn't even notice (or understand) that checkbox in the first place.
Think about a new forum member who has never been on a forum before. A checkbox that says "Remember Me?" makes little sense to them, and might even come across as scarily Orwellian. They are already overwhelmed by all the nuances of the registration procedure. I don't think it's likely that they'd check a box that says "Remember Me?" and they sure as hell wouldn't be able to foresee they problems they'll encounter by leaving it unchecked.
Instead the change caters to the few users who actually don't want to be remembered. (It saves them a single click.)
IMHO, that's one step forward for a few, and two steps back for the community at large.