So what? Since there's no central authority to block transactions or seize funds they'll simply be passed around until any relation with the crime is meaningless with almost everybody in the transaction chain is blissfully unaware that somewhere they were stolen. Then what? If you find the person behind the wallet and seize the "stolen property", you introduce a massive transaction risk that totally undermines the cryptographic guarantee that the transaction is final and irreversible. Imagine the following scenario, you sell a car for bitcoins. The bitcoins come in, transaction is verified, you hand over the keys. Then you try to spend your bitcoins only to be told that they're stolen, we have the serial numbers and is returning them to their rightful owner. Now you have no bitcoins and no car and good luck recovering it.
Imagine if cash was that way, every time the grocery store tried to despoit money at the bank the bank would say "oh no, this and that bill came from a gas station robbery two years ago so we'll return it to the gas station and deduct it from your deposit. The system would crumble as cash couldn't be trusted to really have the cash value it says, even if it's a genuine bill. Everyone with money of questionable origin would pass it off to others who can't and won't verify their legitimity and let others pick up the tab. By all means, if the cops can uncover a whitewashing operation that's fine but once it's passed back into normal circulation again you can't suddenly take away that value.