So, if any customer disobeys those rules and send Bitcoin from the mixer or any sources that the exchange doesn't want, they will block the persons account and the customer could lose their coins forever.
The exchange are doing so to protect their image too based on the government policies, I guess.
So, when they cease those coins, how sure are we that they will take it to the government?
At that point, it is just non of our business and before a regulatory exchange be adhering to such TOS with the users (customers), there must have been an indoor agreement of where the money goes in such cases of users violating the exchange policies.
I think that is how the US was able to acquire the Bitcoins that is to be their treasury reserve.
The governments also have techs that tracks the exchange transaction in the Blockchain while being limited to access users Privacies unless the exchange have to release the needful informations to them.
So basically, the government has an edge to condition the exchange that every tainted transactions of users in their platforms must be under their (government) custody which the public as we don't even have an ideal if the exchange will have a percentage of such seized funds.