I think the time has come -- perhaps it's long overdue -- to distinguish the interests of the airline itself (United, in this case) from those of its employees. Sure, employees don't really enjoy being photographed. We all get that. And they'll pull out all kinds of excuses to stop passengers (and even non-passengers) from taking photographs of them. In this case, the airline has even written a rule that conveniently serves the interests of its employees, though it does little or nothing that I can discern to serve the airline's own interests. In this case, the Port Authority also wants to discourage photography, so all the interests are neatly aligned -- except those of the photographers, of course.
Having said all that, does anyone really believe that the airline would cite a photographer for trespassing because he or she failed to follow a meaningless and likely illegal rule? And endure all the attendant bad publicity? Personally, I find that very hard to believe. If pushed to the wall on this, any sensible airline -- and United is in that category -- will back down. So will the Port Authority, as experience has shown. And the employees will be peeved, but that's just too bad for them.
Bruce