The players don't follow it in regards to NPCs or towns, but implicitly follow it by producing an abundance of gameable consequences.
GMs are often too quick to pull out Asmodeus or Cthulhu or a red dragon and kill them off, spending that dramatic nickel, but most GMs worth their salt throw in a dozen good ideas for every statblock they sacrifice on the altar of a drunken night of polyhedrons. Those GMs abide by it if sometimes by accident, and those who explicitly don't follow it lead short games.
GMs aren't usually particularly concerned with it in regards to players outside the structures usually put in place by the game rules or if you swing that way social contract.
When I've seen GMs sit in on established worlds like the Marvel Universe or another person's fantasy campaign they're almost over-eager to do their part to abide by it, though never to the point of overdoing it and doing more harm than good. I imagine it happens, though.
Generally, players sitting in on another person's PC abide by it as well, striving to do their buddy proud.
Players I've seen running established characters like the Hulk or Panthro have little to no regard for it.
In camping out and in dating young gay men, it's a truism that you should leave things better than you found them. I'm thinking about running a game set in the same world as a game I play in. I'm not sure if I'll go through with it but I do know from earlier conversations that my bud is amenable to this. I've done this before, as have a couple of other people in his regular group. Last time for me it was a one-off, oh-crap-something-fell-through-who-can-do-a-one-shot? kind of thing but I think I did ok...
Thinking about how I rolled last time and what I want to do this time has got me thinking about the campsite rule. So just like last time I'm going to...
Bring everything I need with me. Don't depend on borrowing the giant devil king someone has set up as a campaign's big bad. They might not be down, or they might not want to run the risk of him being taken out like a punk, or taken out "offscreen" at all. So if I decide to do an adventure about a local army defeating itself through infighting, it won't be the army my party is currently facing.
Make the most of my surroundings. In camping this means picking your spot, seeing the sights, and it's the same in games. If your adventure is set in a city, is there one nearby? If it's set in a lost temple, is there one on the hexcrawl that hasn't been filled in yet? What kids of generic monster goon are around? Kobolds? Don't worry about figuring out how to cram Svnerbinvegenjiggiden into your desert adventure. Just use the kobolds.
Clean up after myself when I leave. All you want to leave behind at a campsite are "footprints," the consequences of your party's actions. But while the consequences of the party's actions should be left around for your "host" GM to use or ignore at their convenience, the actions themselves should be self-contained. Don't leave things in the middle of the Two Towers. Scale things down so that you can wrap that shit up before you go. They should have the choice of revisiting your stuff, not be bound to it.
Avoid taking anything off the map. Seriously, you're only borrowing the glimmering spire of Hachgaggle, don't intentionally blow it up in your denouement if you can help it.
Not being greedy. Don't do a whole campaign just to set things up to be awesome for your character, or to change someone's world into something more like what you'd have invented. This is about fun, not personal profit.
Adjusting to the environment rather than insisting it adjust to me. What happens in a game happens. That's the game. No plan survives contact with the enemy or a RPG group. This isn't specific to running in an established game world, this is just good GM advice in general, I think.
I'm also going to break the metaphor by making sure I leave some usable NPCs should your "host" wish to use them. I'm not talking about a colorful shopkeeper, anybody who can't do a colorful shopkeep should stop GMing. I'm talking about Brendar the Homeless, Axe Man of the Wolverines. Targeaux, prizefighting male prostitute. Selindrenn, shapeshifting con-elf. Leave him some guys he can use if he so desires, apart from the big bad of your campaign.
For that matter, I usually try to leave behind one invention. A new spell, monster, piece of lore, or intelligence that comes from whole cloth but which can be dropped into a session independent of whatever the "host" has coming up. He can ignore it altogether or he can have it pop in for the very next session. This is me helping a GM to torture not just his players but to torture ME.
It's not just about running. This is how I try to play as well. I said try. I have a horrible case of Nick Bottom Syndrome, known in the common parlance as being an asshole. But I try to never forget that any particular game is not all about me. Once I've gotten one good moment or laugh, it's my job to try to get everyone else into the action. I don't care about magical items most of the time, better to have someone else use it. Something awesome happening is something awesome happening, whether my character did it or not. When they were filming Ghostbusters allegedly every scene was an unspoken contest to improvise the best line but, once there, they weren't particularly tangled up in who gets it. The important thing is that everything in the final product should be awesome.
It's all just talking about respect in the end. You want to play around in someone's sandbox because you think it's awesome, whether you're a PC or a visiting GM or someone scalping an existing setting. You're playing with these people because you appreciate their contributions to making everyone's time awesome and you want to do your part. Get your friends laid, get their character laid, get your setting its own Crawling Throne.
Which is like RPG... idea... sex.
In chair form.
I am....fucking tired, I think I had a point.