I assume users were receiving bitcoins that were mixed and more likely some users would end up with bitcoins that were used for black market and other illegal activities.
I have euros, dollars and pound sterling that have most likely at some point been used criminals. Simple evidence:
most bank notes contain traces of cocaine, and there are countless other crimes that don't leave physical evidence on the money.
None of this is a problem for me, because money is fungible. And no government would dare take that away, because if they do, it will be the end of money as we know it.