Unfortunately, for centralized mixers at least, you're wrong. In the US, you're prohibited to run a non-regulated money transmitting service; and that's pretty much what a good Bitcoin mixer is. Anonymous and it involves and intermediary who will transmit money. However, that does not apply on every country, and it does not mean decentralized mixing is illegal.
If centralized mixers should follow the Bank Secrecy Act and register as a money transmitter, they have to follow anti-money laundering and know-your-customer compliance. But is that possible? If they follow it, that means they are no more mixers but another thing else.
But I am wondering why mixers are allowed on sites like bitcointalk and no regulatory ban.
Just like I said, gambling is not legalized in all countries in the world. Cryptocurrencies are not legalized in a countries of the world, there are over 10 countries in the world that completely ban cryptocurrencies. Because something is not legal in one country, that does not mean it is illegal in another country.
We can just take it that mixers are not illegal. But if they see it having illegal mixing, there is possibility that it would be taken down.
sometimes i often think of the government law as nonsense. they can just declare anything illegal if they cannot impose power on it. i'm not saying laundering is legal and mixing is illegal, but how come they assumed someone is committing illegal actions when they just want to hide their identity? does that mean because they couldn't impose taxes to those who are mixing? are they also considered tax evaders? non of this makes sense when someone is trying to enact law just because it is convenient for them. the only thing that made sense to me is that when you follow the regulations of the government it is impossible to hide your privacy.