That's why you use mixers. Good mixers will completely break the chain between inputs and outputs.
Why are mixers legal? The only purpose I find on them is to money launder. They are literally useless if you don't have anything to hide.
I don't see anything called useless here, mixers are for those who want their identity secure without a trace but for money laundry this depends on individual aims. personally i used mixers for safe identity, not because am into money laundry of a thing. Bitcoin Mixers play vital role in keeping the anonymity of any Bitcoin holder who want his identity safe.