The offensive foul call was certainly a unique one, that you never see called. However, are we so sure that it's *not* an offensive foul?
Think of it in reverse: if an offensive player jumps for a dunk, and a defensive player slides over to get in the path of that jumping player to create contact, it's a defensive foul. Doesn't it make sense, then, that if a *defensive* player jumps, and an offensive player slides over to create contact, that it would be called an offensive foul? After all, it's an offensive player initiating illegal contact with a defensive player.
It's certainly a novel call, and I agree that refs can't all of a sudden change the way the rules are interpreted midway through a series. However, maybe this is the way officials *should* be calling games.
(As I remember the play, Pierce took a step to the right to make sure contact was created. If he'd stayed in place, of course it's a defensive foul.)