Completely unacceptable! The only reason to close out early would be if all ticketed passengers have checked in and have boarded.
I'd be curious to know if the flight was oversold and if anyone else missed the flight. If you can find out if the flight was oversold and if it was indeed full at closing, then I think AA owes you a huge letter of appology and a nice compensation voucher $$$$$! This is surely no way to treat any customer, let alone their best customers.