I see almost none up here (Alaska), so some places are as yet unspoiled by litter. Just the ocassional wrapper that hopefully just slipped out of someone's pocket.
People seem less likely to litter in a clean place and when it's first bit anyone's seen in 5 miles, most anyone will pick it up. It's when the balance of cleaners to litterers gets too out of whack that those in the middle start to slip.
I was picking up trash on trails long before Scouts, but what I learned in Scouting was to the "police" the campsite for litter. Little of it was ours, but we cleaned it all up. Now my grade-schoolers expect to do the same thing on any cabin, camping or BPing trip.
My two gripes: Ammo cartridges. I don't care if you don't reload, pack it out. And any foil or glass in a fire. Tin cans to burn out the smell, okay. And if you want to burn paper or plastic, do while you have a hot fire and it will combust decently.