Binance has already announced that KYC will be mandatory for the users from the United States and they are working to create a separate trading platform for the American users. Users from other regions don't have to undergo KYC as of now, but as the governments and the FATF get increasingly strict with their proceedings, I believe that it is just a matter of time before KYC is made mandatory for everyone. Same goes for BitMEX as well. They can't challenge the feds. We all know what happened to BTC-e, after they did something similar.
the situation is fundamentally different with a crypto-only platform. when BTC-E went down, they lost access to most of their fiat funds held with banks and third party payment processors. the losses were limited to fiat money. and one of the two charges was for unlicensed (fiat) money transmission.
in spite of the feds mirroring their servers for 6+ months, BTC-E retained full control of all crypto funds, which they used to partially refund users and resume services under a new brand.
this is the context we're talking about. with no customer funds held in bank accounts, and with properly distributed servers, the feds may be able to do little more than seize the domains of services that don't comply. tbh i wonder if the USA is interested in that because it will just make them look weak and impotent.