This claim is substantiated by Bestmixer's page where it lists reasons why people should be using mixers in the first place. They basically advertised their service as a way to evade KYC and AML procedures with crypto, and the authorities justified their attack based on that fact.

Mixers are otherwise perfectly legal (which was acknowledged by a blog of McAfee, a firm instrumental in the takedown) and I don't see that changing in the near future. Lawmakers don't ever talk about mixers and I would be surprised if majority of them knew mixers even existed. I could see mixers getting on their crosshairs if a single provider gets big and popular enough, but the service itself is so niche that even the most well known ones remain obscure to the general public.