Etiquette. How long has the stand been there? Is it showing signs of recent use?
OK. It is on public land. If the stand is new I wouldn't use it. If the stand has been there for many seasons I would have no problem using it. One cannot erect a stand in a tree and for perpetuity expect that the stand be exclusively theirs. If you have a problem with that then don't use it. On the other hand, like you say the stand is in an excellent location. Again, if the stand has been there for more than a season then that location is not the exclusive location of the person who put it there. For example, I may have been hunting that exact location for years and years. Every time I come and go I bring/take my stand with me (it's a portable stand). Now all of a sudden I come back and there is a bunch of two by fours and plywood nailed to the tree where I have been hunting for years. Guess what, the location is not owned by anyone. I'll put my climbing stand on a tree right next to it if I feel like hunting that spot. We have to get along in the field and forest. If you hunt public land you have to expect some intrusions. Be reasonable.