The opposite problem from cutting through someone else's campsite, is expanding your campsite to block a public right-of-way. At some of the Pines campgrounds, for instance, there's a four-foot-wide public path between the "campsite boundary" posts and the fence above the river. I think the path is there so people can walk along the river without intruding on any campsites, but a few times I've had to step over someone's chairs or detour through their campsite in order to continue on the path.
At Camp 4, there's a path that starts at the phone booths in the parking lot, runs straight back between the parking lot and the campground, and becomes the Upper Yosemite Falls Trail. One time when I was walking toward the trail, two kids were taking a lunch break there. One was sitting on a rock on either side with their food and gear spread out across the path just like a picnic table. Picking my way through the mess seemed like way too much trouble so I detoured through the parking lot to get around them.