They literally aren't, though. They are the airline's private property, and you've entered into a contract of carriage with them to temporarily have access to their property under the terms of the contract. To some extent these terms are circumscribed by law, but in the absence such law, you are bound by whatever terms you agreed to in the contract. If you don't agree not to film in the plane, you're free to not accept the contract.
They literally aren't, though. They are the airline's private property, and you've entered into a contract of carriage with them to temporarily have access to their property under the terms of the contract. To some extent these terms are circumscribed by law, but in the absence such law, you are bound by whatever terms you agreed to in the contract. If you don't agree not to film in the plane, you're free to not accept the contract.