I'm sorry I don't like taking down a tree stand and trying to hump it out in the dark. or go back for it in the dark after humping out a deer. And if I got nothing I use it in the morning, and I'm not humping out and setting up a tree stand at 3am for a morning set.
Besides I don't know about you, but packing in 60 pounds of #### through the woods makes me sweaty, and that doesn't go well for a hunt. Hence why i set up a tree stand in the afternoon, go back to camp, eat wash up get changed out of my "camp" clothes and into hunting attire that i try to keep clean and away from fire and food.
how do I know the guy on foot wasn't their first, because I would have been there in the morning scouting, setting up the stand in the afternoon, and heading out a couple hours before sunset to occupy it. If I found someone in my stand that means the would have come in after I set up the stand while i went back to camp to eat, change clothes, grab my gear and scoot back.... So yeah I would make a stink about it, the weasle just wrote off an entire days work for me, wrecked my hunt, and is using my property without permission.
cleftwynd has the right idea. you see a stand up, especially one that has been erected recently, move along find another spot. Even if I scouted an area I want to hunt on foot in the morning, come back in the evening to find a truck, tree stand or evidence that someone else is already there or now invested in the spot I move along.
As for fellow hunters, if i find a camp in the area i'm in I stop to talk to them, maybe share a beer. But ultimately it's so we can talk about where we are hunting and avoid stepping on each others toes in the field. a Friendly "I'm planning on being down this cut line tomorrow morning" is far better then a heated argument on site when you accidentally bump the other guy during a set because who knows maybe he planned on using the same cut line. A 5 minute game plan talk works well to insure everyone has a good hunt.