It is just like what Bittorrent and Piratebay are doing as far as file-sharing is concerned. They are not breaking any laws by sharing the files themselves. But the users can make use of the software/website to share files that can breach copyright rules. Megaupload of Kim Dotcom was shut down due to the same allegation. It didn't helped that they never did any piracy on their own.

Let's just get it straight. I don't have any doubt that the majority of the coins that are mixed using these sites are "dirty" (i.e originating from exchange hacks, scams such as PLUS token, revenues from dark market drug trafficking.etc). Unless the owners of these sites can show the authorities that they are taking precautions to prevent the flow of such dirty coins to the mixers, they will be considered as illegal by the government.