It was most likely because of his no-trade clause.
Ok, here's a good waiver question. If a player has a no-trade clause, and he gets claimed, can you just let the player go to the other team? I know you can just give them up (a la Randy Myers), but I wonder how that works if it's a waiver deal. It wouldn't "technically" be a trade...
I thought I had the whole thing figured out, but that's a new wrinkle.