It does seem a bit unfair to place a flight attendant in what can be a very awkward and no-win situation. If they determine that someone is in violation of UA's Exit Row policy (which is compliance with the FAA regs), they can observe the policy and force the customer to move (creating a negative customer experience) or ignore the policy and risk whatever consequences there are for doing so. In any case, I would have a hard time faulting a f/a from enforcing the exit row seating requirements.