While I understand what you're saying, keep in mind that mixers can be used to break the link between two addresses just for privacy reasons. I don't like the idea of banning addresses because then we're choosing what is a "good" or "bad" Bitcoin, but I would not completely blame a mixer for staying on the clearest legal side by avoiding addresses which are known for having been involved on scams.
i.e You ban a famous address that was used for ransomware while allowing Bob to get some privacy on-chain.
Yeah, currently, mixers have no reason to ban specific addresses. Mixers haven't cooperated with governments to prevent money laundering, and they guarantee user anonymity, so they won't interfere with issues related to identifying BTC owners.
Well, in the future, we could totally have a "legitimate" mixers service, operated by governments with the goal of serving users anonymity needs while adding a feature to ban accounts related to crime. Maybe it would only be used for users to hide the number of BTC in their accounts from others, while still trusting governments. I can't imagine how this new type of mixer would work, or it might not even exist because it wouldn't be trusted by users ^^