No, he wasn’t trespassing. A trespass is occupying property without the legal right to do so; this passenger had a valid license to occupy the property, and United did not have the legal right to remove him under the reasons stated. UA ground staff confused their authority to deny boarding with the authority to…

They weren’t police, they were hired security. If you want to get him off the plane and you think he’s breaking the law, call a cop. What these guys did was simple assault and battery. Charge them for that crime and I’ll entertain the trespass charge.

Alright, this is where it gets murky. The FAA rules require compliance with the crew, but the crew cannot make you follow directives that violate the Common Carrier Agreement (which this does). I believe the airline is ultimately at fault. They are the ones that performed a passenger removal that is outside of the FAA…

Read the Carriage Agreement on your ticket next time you fly. It applies up to the point of boarding. Once the passenger passes through the gate, the legal situation changes and the FAA rules take over.