Jurisdictions can issue evacuation orders and curfews. Police can in many cases remove you for trespassing.

The United incident was a total disaster for United, they were incompetent and doubled down on that incompetence because of policies driven by their bottom line. The “police” that removed the trespasser were no trained and were also incompetent.

None of that changes the fact that if the captain of the airline thinks it’s a safety problem he can remove the passenger.