It is indeed illegal for the government to know that there are mixers for bitcoin which is money laundering where the mixers remove any trace of that cryptocurrency like bitcoin. There is nothing new here to know that there are mixers since even with the fiat currency, there is also a process to clean your dirty money.
There is crime everywhere and I don't think they should close the bitcoin mixers because it helps man of bitcoin users to mix their coin and protect their identity. Bitcoin mixers is just one of the many ways to make a crime, I don't think they should close it.