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.
If you are going to as this, you should also ask why Bitcoin is legal. That is because one of the main characteristic of Bitcoin is provide you with complete anonymity and privacy with your Bitcoins. It will be upon the realization of the person whether to use Bitcoin for good or to use it in frauds. That is the thing I like about Bitcoin, it gave me the complete privacy and freedom to transact if I want anytime, anywhere. And if happens that we read a news about this kind of hacking again, it is not Bitcoin mixers to blame but the person behind it. That is why we should keep our Bitcoin's safe and be cautious with it.