Yes, UA should make this more clear. Could they text reminders to those who sign up for travel alerts?

<Speculation based on observations> I think the REASON that UA is so strict about the check in is because the ramp agents have to play one heckuva game of Tetris with everything going into the baggage hold. And it's not just for baggage either. Mail, shipments, critical kit of spare 737 parts, etc. all pack every square inch of the hold, to the point where on my last Island Hopper flight UA removed (and presumably rescreened for TSA-prohibited in-cabin items?) 12 suitcases out of the overfilled luggage hold, to put into the cabin and store in the overhead bins! The opposite of any other direct flight between two US airports. As for cutting off passengers who don't plan to check bags, my guess would be they can't risk the game of Tetris being rearranged by someone who has to gate-check a bag, as the amount of work that would have to be re-done could delay departure. </Speculation>