It's very likely that they are rejecting mixed coins because the US government has privately ordered them to do so ("if you accept recently-mixed coins, then we will investigate you for financially supporting terrorism"). So in a way, coin mixing is already illegal, but only for regulated companies to accept, because that's the only way the government can fight the mixing of coins.

If that seems absurd, consider that Facebook gave US agencies direct access to delete posts from Facebook. The US very likely coerced Facebook into giving them access (e.g. "If you don't let us delete posts by people we deem to be terrorists, then we will investigate your company for possibly assissting terrorists").