Banning US citizens is clearly in the Terms of use on WEX.nz for all to see. I would call that set in stone.
Speaking of banning US citizens, it seems that Bitfinex is terminating trading, deposits, and withdrawal functionality for U.S. individual customers by November 9, 2017.
I wonder how many other crypto exchanges will take that hard line?
Remember, though: BTC-e was indicted for operating an unlicensed money service business and fined $110 million for it. Bitfinex has been doing exactly that---with no MSB licenses in any of the 50 states---for something like 5 years. They are pulling out of the market now that law enforcement moved against BTC-e and regulators (like the CFTC and SEC) are ruling on ICO tokens. But we'll have to see if it's too little too late. I suspect that the various US agencies are months or even years into investigating Bitfinex. Pulling out of the market now doesn't erase past violations.
This isn't the wild west anymore. It's possible that exchanges might get away with flouting the law in the early years. To do so now is suicide for any company that wants to remain in existence five years from now. The message to exchanges operating in the US is clear: get proper licensing, or get out.