It's the enabling part
Mixers cater to money launderers 
They are participating in the act
And laundering money is a crime 
(It can also be automated not just manual)

If I trade in your tokens for other tokens (or fiat for fiat) without asking how you obtained it, and it was obtained illegally, then I am actively engaging in a crime. You would have to prove beyond a reasonable doubt that you had no knowledge... which is very hard. 
(I don't make the rules)

The crime is absolutely financial, even mob stuff, otherwise there would be no need to hide the money.
Hitman for hire - crime involving finances 
Robbery - crime involving finances 
Drug / people trafficking - crime involving finances 

The crimes are committed to gain wealth 
There's not really a distinction 

Furthermore, transactions aren't supposed to be private in the eyes of the government. One reason being taxes.

People say that bitcoin offers privacy,  if true, there would be no need for mixers.

If the feds are going to assume that privacy is directly linked to crimes, there would be a lot more companies going down.
Which remains to be seen. 

(I'm guessing that's what this was about)

They probably will anyway to try to combat ransomware... it's all paid in bitcoin. 

I'm not saying people shouldn't be able to have privacy. 
It is what it is. 

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