That is possible, but the problem is the time it takes to disseminate the knowledge of the crime is longer than the time to trade the bitcoins. So if the thief immediately exchanged them with some merchant right after the robbery, then sometime later, the coins were marked as tainted, the merchant would suffer as well, since they're now holding the coins.

This would create a chilling effect on trade since now you have to worry about the legitimacy of the other party, which brings us right back to the problems we have with credit cards, etc.