I see far to many people defending the exchanges engaging in these behaviours in this thread... Really... I'm astounded that the general consensus isn't shunning these exchanges and making a public list as to which exchanges should be avoided at all costs.
I tought about a relatively analog situation... It's actually the other way around from the post made by the OP, but it's equally valid.
The situationPerson A sells guns on openbazaar accepting BTC
Person B sells his kid's second hand toys on openbazaar (a digital garage sale) accepting BTC
Since openbazaar uses a HD wallet for the seller, the buyer (that can potentially be law enforcement or a criminal) is now able to follow the trail of unspent outputs untill they are exchanged on one of the many KYC exchanges. If the buyer is Law enforcement, they are now able to get a warrant and tie the identity of the seller to the sale. In case the buyer is a criminal, he can wait untill the exchange is hacked, he can hack the exchange himself, he can blackmail an exchange operator, he can bribe an operator,...
Bottom line is that, as a seller, it doesn't matter if your client is the police or a thief, you have to be realistic and know that in the future, it might be possible for either seller to find out how much BTC you hold and where you live.
In case of the gun seller, this is good if law enforcement can tie his identity to his sales (so they can arrest him)... In case for the garage sale it's VERY bad (since the thief now knows how many BTC person B holds, and knows where he lives).
Now, i hope i've made my point that both person A and person B have added benefit when mixing, coinjoining or exchanging to an anonymous coin. I hope this makes it clear that it's NOT the act of mixing/coinjoining/exchanging is illegal. There are, in fact, perfectly legal reasons to mix/coinjoin/exchange. It's NOT up to the exchange to lock users because they want privacy. It's up to law enforcement to track illegal gun sellers down, and only get warrants for people that are doing illegal stuff. An exchange, even if it's looked upon as a financial institution, is NOT a valid form of law enforcement. They do not have the right to meddle, track, block, ban,... The only thing they are allowed to do is keep the KYC information SAFE untill law enforcement comes around with a legal warrant to request this information. A warrant based on concrete evidence that the person that's being investigated has a high probability of having done something illegal, a warrant signed by a judge.
If exchanges are forced to do act like law enforcement by the law of the country they operate in, we should switch to exchanges that are located in countries where the laws aren't completely retarded. If exchanges took it upon themselfs to play "world police" they should be dropped, shunned, avoided, sued,...
Would you like it if you FIAT bank decided to block your account because you worked for a brewery, and since drinking and driving is illegal, they no longer accept the money of people that have anything to do with alcohol or tobacco? Would you be OK if they blocked your bank account because they saw you walking out of a pub at night? Would you be OK if they blocked your account and did not want to release your money before you gave them an absurd amount of personal information? Would you be OK if they plain blocked your account and did not give you any way of getting your money back?
The answer is: NO, you would not be OK... If this happened to you, you'd be searching a lawyer by now, you would have contacted the newspapers, you would be calling your relatives to withdraw all their money from said bank... Why on earth do people think this behaviour is not OK with FIAT, but perfectly fine with crypto?