I had heard that One major US carrier had a stack of Tarmac delays filed against them and that the doors being opened (but seat belt sign on) meant pax could not legally leave the aircraft (without risking the $10,000 fine)?
“It is a violation of 14 C.F.R. 121.317(f) not to have your seat belt fastened when the seat belt sign is on. Civil penalties up to $10,000 could be assessed. “