Originally Posted by
WBurcham
If you've got a confirmed reservation and are at the gate less than 10 minutes before departure, you SHOULD NOT HAVE BEEN OFFLOADED.
The domestic C of C makes reference to this, at 15 minutes, but your point stands.
From U.S. cities, you must be at the boarding gate at least 15 minutes before your scheduled departure time with the exception of St. Croix (STX) and St. Thomas (STT) which require 45 minutes.