If USDT is no longer equal to USD, then you can't trade BTC/USDT anymore. Why don't those exchanges just switch to USD instead of USDT?
because usdt is in theory just another alt so it avoids a whole bunch of banking and legal problems. poloniex is pure alt with no fiat possibilities. that's tether's problem. they'd have to spend millions on compliance to switch to usd.
On a functional government it would not take long to close that loophole, expanding regulations beeing in effect for the dollar onto anything else that tries to act like a dollar, too.
Thank heavens they are busy elsewhere.