They are trying to avoid prosecution by the SEC with a rule that U.S. people can only sell BFX tokens, but they cannot buy BFX tokens. Bitfinex will argue that they aren't allowing US people to trade them because they can't buy and sell them. I don't think that strategy will work, the SEC will probably prosecute them anyway.
I'm pretty sure that strategy will blow up their face. The point of contention, I'm fairly sure, is the
issuance of the unregistered securities. These are pretty clearly some kind of bond (whether convertible or not), so I can't imagine a sound legal argument that they aren't securities (at least under US law). I'm guessing the SEC will eventually come down on them (perhaps even shut them down), but US agencies take months and months to build a case and act.