" if a full size helicopter made an emergency landing on the property, can the park refused to return the helicopter to the rightful owner."
That's true, but not the best analogy that guy made in the video because he tied it to damage to the lawn or trees or something. A better example would be if a beach ball accidentally bounced over the fence into the water park. It does not become their property, did no damage. The owner of the beach ball has the legal right to recover his property. If he called the police they would ask the park owner to retrieve it for him, or allow the owner of the property into the park or the police themselves might retrieve it. Not always about "damages" the park suffered.
So I think in retrospect, the smart thing for that kid to have done once they denyed gving it back to him would have been to call the police that moment, while he is standing there. Even if it was hard to get to in a tree, they could have used a long pole to knock it out. Sure, maybe it would break, maybe caught it but that's not the point. I bet the owner of the park would not have been such A-Holes about it if a cop was standing there trying to resolve the issue.