They mention VPN in the letter. So it is fine unless they figure that out. I would hate to give them an excuse to basically confiscate my funds. Since it is "illegal" , US citizens can't expect any help from their government. Of course the argument can be made that bitfinex is just pretending to care, to get the US government off their backs.
There are some other good exchanges out there. Bittrex which is a US company and is trying very hard to stay in compliance. Which is a hassle since they keep delisting coins, but you can probably bet they won't steal your coins from you. Binace is a great exchange and very low hassle. They do great things like give you your GAS if you have NEO. They support most forks and you might even get the occasional air drop.
It does not seem like it would be worth the risk. Also I would not be surprised to see the EU start giving the exchanges some grief. Seems like crypto's are now on their radar.
The worst part about living in the US as far as crypto's is concerned, is that most ICO's are off limits. I do know that a lot of folks in the US use VPN's for that. But there is a difference between putting in a small amount ETH for a one time transaction and maintaining a membership on an exchange.
Your question reminds me of a line from a Bob Dylan song, "everything is legal in Jersey, as long as you don't get caught!"
You nailed it. The explicit purpose of these emails is to give the appearance of compliance. If Bitfinex was really concerned about US citizens using their platform, they would have started blocking all IPs a long time ago, but they DIDN'T!