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.
Will they pass them around? Enough to blur any relation ship? In a secure way that never leaks any identity?
(oops, one of the exchange I sent money to managed to record my IP address. No matter how much I keep mixing downstream, part of identity are leaked here)
Remember that they have adversaries like government who (as recently proven for the NSA, for example) have quite a few ressources.
A single policeman might not be able to pull enough data and analysis.
But if goverment suspects that some big danger as possible ("pedo-terrorist pirates!" threat, or more realistically: juicy corporate spying opportunities
It's not impossible for the thief to manage to get out un-identified. But it requires being particuliarly smart.
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.
Cash *does* function this way (a bit): bills have serial numbers. Of the grocery stores deposits a bill with a known serial number on it, police might show up the next day asking for the CCTV suveraillance tapes, because that serial number happens to be a bill passed through the hands of known drug kingpin/terrorist/pedophily ring leader/etc. do it enough with enough of such incidents, and you might get a vague idea of the identity of the people you're looking for.
Unless the criminals have been absolutely perfect in their laundering and have managed to never leak any info (i.e.: by the time the known bill are flagged, they're in the hand of complete random strangers).
Google for "Ransom bill reappear" type of news reports.