Thanks for putting that exchange to light. I realized Cryptsy doesn't seem to know how this works. However, their last support email that I received, after BigVern's reply above, was
"I promise you that we don't want to steal anyone's coins...
I'm talking to Vern about this now, and will let you know the solution.
"
(I realize steal was a strong word, but I don't think there was any other way to get their attention!)
Even though exchanges might have to go through an additional step, I think it is only fair that they make the distributions to their users (yes, I agree holding the coins yourself is the best way). Same goes with other PTS exchanges like Bter. I don't see a terrible cost to the exchange, honestly. The procedure should be fairly straightforward. Worst case, they could take a fee of 5% of MMC and give the rest to the users.