In the UK, the high seats would be regarded as private property, generally they are erected by the syndicate that controls the shooting rights on the property. High seats here have to conform to strict health & safety legislation so you cannot just knock them together out of anything.

The owners would be most upset if people were messing around with their high seats and ended up injuring themselves. There are plenty of no win, no fee lawyers just waiting for an idiot to fall out of one and break something. Best to steer clear of a high seat here unless specifically told you are welcome to use it.