Because it doesn't protect anyone. If a criminal sees that his coins are refused to enter the coinjoin, he will simply mix them elsewhere. The coins aren't returned back to the victims, and likely never will.
So what sort of mechanisms should zkSNACKs put in place to defend these victims from these criminals?
When someone infringes on another person's freedom, there must be a mechanism in place to protect that individual if they cannot defend themselves.
There's Bisq, a decentralized, self-custodial exchange.
Bisq uses trusted third parties who escrow, it's not self-custodial.
I understand you're fixated on dictating what people can and can't do, but the simple fact that people use them is undeniable proof that they provide a service. For example, there have been mixers in the past that charged no fees at all, not even the mining fee.
Hey BlackHatCoiner, my wallet is now a "mixing site", send me your coins and I won't charge you any fees.