Simply put, tree portable tree stands are not illegal, what is illegal is any damage that is done to a tree by a climbing tree stand. If the bark is cut though to the cambium layer, it is considered that damage has been done because a cut that deep offers access to insects and diseases that enter trees through damage to the bark. Not choosing a thin barked tree such as a beech or birch lessens the chances of doing damage.
When you are up to your butt in alligators, it is hard to remember your intent was to drain the swamp. Stay focused!