It seems like a pretty cut and dry case. The purpose of a bitcoin mixer is literally to hide the source of funds, which in any other financial transaction would be classed as laundering. Anyone who owns these sites know full well that the majority of their customers will be sending dirty money through their system in order to try and clean it. Every single mixer that gets big enough will end up being prosecuted like this if the US authorities get hold of the owner. The feds also get to seize large sums of money and also figure out lots of customers who might have been trying to avoid taxes - people should stay far away from such services or they might get a knock at the door in future.