Are you referring to out of season or during? I have seen stands left for both. The year round irritate me, but the ones where they leave it overnight I take in stride. Just try to figure out their entry and exit path(s) and go from there.
Public land here in the lower peninsula of MI is a crap shoot anyway, most pressured hunting according to some.
Same here in PA, our hunter densities compete with anywhere in the world as far as being most crowded.
A few years ago the Game Commission instituted a rule on game lands that stands must be tagged and removed X days after the season. It hasn't done much, it is hardly enforced to my knowledge. I see more and more stands every year.
The argument is always "I don't want to hunt where the other guys are hunting". Well that argument is bullshit. Some of these litterers actually manage to leave their stands in pretty good spots. You want a good spot, wake up earlier than me and beat me there. Don't leave your shit in the woods and expect that to reserve your spot.
I've heard too many stories of confrontations arising in such a scenario. I had one the first day 12 seasons ago...first day, public land. Went to a spot that I had a few close encounters in archery. I was proud of this spot, it was my first year hunting away from my dad and I found it all on my own. Climbed my tree in my climber, 1 hour before shooting light. Right before shooting time a dude saunters along and climbs a ladder stand 50 yards away. Shit. I had no idea that stand was there, this was in pretty thick crap.
Well I stayed because I knew I was in the better spot and as expected I whacked a good buck at about 8 AM. This asshole comes and reads me the riot act because I moved in on "his spot" that he had been hunting for years. I was in the tree 45 minutes before him. Mind you I am 17 years old at this point and weighed about 120 pounds soaking wet and he is like 50-something. I was even apologetic at that point. No attitude was given. At this point I was already writing it off as a rifle stand but you bet your ass I was going back there in archery.
The next year in September (the guy told me he only rifle hunted) he put a stand in the exact tree I shot that buck out of, blocking me from even archery hunting that exact tree. I knew it was his because it was the same homemade design as his other stand. In a fit of immature rage I tore effing stand down and scattered its components around his original ladder stand. Kind of regretted that because then I was too much of a pussy to even hunt the general area and I lost a good spot. So now I just give the litterers a wide berth. Not so into confrontation and a decade of adulthood tends to improve your judgement a bit. It's just a violation of the golden rule. We all share public land and claiming spots is an asshole move.