since Poloniex is based on US already, then it is best if you assume that they have reported your history to the different government agencies. even if they haven't it should always be a possibility that they will do in an instant upon request.
I by default assume that my personal information and trading history has been compromised by regulators. It just isn't worth it to avoid taxation. One way or another, they will catch you off guard and still make you pay.
It's debatable and very subjective whether or not taxation is fair, but there isn't a way to get around it when you're using centralized services and they have all your personal information.
At the end of the day, fiat exchanges have turned into a slave of their banking relations already, so what would prevent the same exchanges from handing over your information to regulators in order to remain operational?