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.
Mixers are legal because it is suppose to be used in legal way, hiding your real identity and for privacy of transactions. Some people are just taking advantage of it using it to launder money and even scam people without getting traced using mixers.