This mixer was penalized for running an unlicensed MSB. This is far more about that then it is about banning privacy technologies generally. For traditional Bitcoin mixers as in this case, someone receives money from users and then transmits money to many users. This is money transmission and requires registration with FinCEN and sometimes requires registration with states (though some states have exemptions for completely crypto to crypto transmission that doesn't touch USD or other fiat).
Mixing in this case is interactive where there is a clear money transmitter. In Monero's case, the ring signature "mixing" (mixing is a terrible/misleading way to refer to ring signatures) is non-interactive, and there is no intermediary (eg: a mixer) acting as a money transmitter. Thus, there is nothing to fear from this specific enforcement action.
I'm happy to answer other questions as well. But for money transmission to occur, an intermediary needs to accept customer funds. For a Monero transfer, there is no intermediary. Someone could build an MSB on Monero itself which would require registration, but using Monero to send funds directly to a merchant for one's own purchase, for example, is not money transmission.