Do you have a cite for this? Because the last time I asked if this was the case, Mt.Gox said no. I pointed out the practical problems with such a policy and they denied they had any such policy.

And I don't see how they could possibly have such a policy. For example, if they retain the right to claim some coins are tainted and not as good as others, presumably they have to extend to others that same right. If I send them Bitcoins I don't consider tainted and they reject them, what do they do? Send me those Bitcoins back? What if I now consider them tainted? (Which is, IMO, justifiable. Since Mt.Gox has now declared them tainted.)

If Bitcoins are not fungible, lots of things start to break.