they even plan to designate mixers as financial institutions regulated under the bank secrecy act:
Can't say I'm surprised by that in the slightest.
What I would be interested to know is how an exchange or other service that has to yield to this would treat coins that come either from mixers that ignore this or are mixed crowd style a la Coinjoin. Would they be automatically obligated to reject them? Will there be a point where you need a completely unbroken trail to go anywhere near a centralised service?
That's the million dollar question. For now, nobody really knows. To my knowledge, Gemini is the only service that specifically prohibits interfacing with mixers in their user agreement. I don't think they'll be the last, though.
Exchanges will likely take a risk-based approach, with a spectrum of different policies. The worst of them will do as Bitstamp already does -- demanding proof of source of funds for every satoshi deposited. Others may only flag accounts that deposit specifically dubious coins that are sent directly from DNMs and things like that.
It's same old fungibility question. Where does it begin and where does it end? I can see it becoming an impossible mess unless everyone's sensible.
Well, what's sensible?