Mixing services simply further obfuscate 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, but 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, the "mixing" only protects a user against a greedy tyrant, no matter what that tyrant says.