I'm not aware of any situation where you can legally be involuntarily strip-searched without being arrested. Maybe by customs but I'm not sure.
Personally, if the flight crew instructed me to strip naked or leave the plane, I would just leave. I can't foresee any situation where I would physically fight the staff for my seat, or recommend anyone else to do so.
I know Americans are a bit high-strung when it comes to "property rights", but here's one example: It may be your property all you want but you can't call the police to kick your tenants out because you decided you don't want them there any longer. IF there has been a contract violation, you can go to court and eventually get an enforceable court order, but if you call the police and tell them to kick your tenants out because you just changed your mind about renting it to them, they're more likely to laugh in your face.
That is what the police should have done in this case. Once they ascertained that the passenger ticketed in his seat and was not a safety issue, they should have said "sorry, this is a contract dispute. You can take him to court."
Edit: Note that after kicking him out he was not arrested and has not been charged with any crime that I've heard of.
I think both of us have a very strong sense of property rights, only we disagree who to ascribe them to. I think it belongs to the airline, so if the man wants to create a dispute with the airline, he should be the one to take it to court (and he probably will.) From my POV, you are saying the man has property rights over the seat because he had a ticket for it and had his arse is in it.
We are not talking about a random piece of property being trespassed upon here. We are talking specifically about a situation where the airline is licensed to take money from the public in exchange for letting people put their "arses" in those seats.