The US government decides what checks a financial institution must perform when dealing with US customers. In this case, BitMEX knew there were serving US custo

The US government decides what checks a financial institution must perform when dealing with US customers. In this case, BitMEX knew there were serving US customers while not performing the US-required checks. You, as a non-US citizen, can do any transaction you want with BitMEX. BitMEX, as a company with US presence in NY, must make sure that you are not a US citizen (maybe based on your declaration that you are not one).