You should take note that not all of their clients' trading history are being submitted to the IRS for review, the order for them was to submit "random" trade reports or unusual amounts of trades coming from their clients. And yes being a U.S. compliant exchanges I expect them to be one of the companies being required to submit trade reports.
poloniex has said repeatedly that they do not do
any tax reporting. i'm sure if the USA government sends them a subpoena for your data, they will comply, but that is a very rare situation. in the regular course of business, they do not send 1099s to the IRS like coinbase does.
If that is the case then I doubt the authorities would even need a subpoena just to know their clients' trading transaction. Subpoenas are only served once there is a lawsuit filed in the court whether it is a civil case or a criminal one which in this case the IRS wouldn't need since they are just doing regular background checks if the U.S. citizens that are trading in crypto are paying their taxes right. The IRS and the SEC might cooperate in this matter and just put up some kind of regulation/requirements ordering these crypto exchanges to make the submission of these trade reports legal.