Mixing should be protected too, but it's important to distinguish between mixing and coinjoin. They were not violating MSB laws, because they were not taking custody of the funds. They were only coordinating peer-to-peer coinjoins.
Discussion
Good to know. Whatever it is, it is a service that further obfuscates identity in Bitcoin, which is too similar to "money laundering" for the three-letter agencies' taste. The fact is that money laundering is not itself a crime either, but only a technique to hide other crimes.
Theft, fraud, and extorsion precede or are followed by "laundering", and the "laundering" doesn't make the crime worse but only justice more difficult to achieve, which one may call obstruction only in the case that another crime has been or is being committed. (Note: a crime anticipated is not yet a crime. That's another path to further tyranny.)
If no other crime is being or has been committed, "mixing" and "coinjoin" only protect a user against a greedy tyrant, no matter what that tyrant says.