Um maybe because if they are left in and the tree grows around them after 20 years, those metal pieces can be completely obscured and when the guy at the saw mill goes to cut lumber they can do a number on his saw. If the screw-in steps are removed at the end of the season I don't see it hurting the tree or the value of the lumber much. On state land I doubt every joe public would remember to take out theirs, most likely reason they are not allowed.