Bad example because in this case United and passenger entered an agreement for the passenger to be on United's property.

That's like saying you leased your house to a tenant for a year but then six months in you find another tenant that you want so you call the cops on the first tenant. Doesn't work like that (well at least where I live).

A contract is a contract and nobody is tresassing here.

That's a civil matter, they're absolutely allowed to kick you off their plane if they want, regardless of what they told you before.

Sure, they're breaching contract, but that's a civil matter. It's completely unlike residency, as well, because that's got special protections, and as many times as people say this has special protections too, it doesn't.

Right, except, like, it does, because airlines are regulated too, by the DoT. Actions here likely constituted a breach of 14 CFR 250.2a

Unless you paid to remain on said private property, and they are bound by contract to let you stay.

That's literally not true.

To elaborate just a bit, breaking a contract isn't against the law, being "contractually obligated to let you stay on the plane" doesn't mean they can't kick you off the plane, at all.