I've skirted the rules on that a few times. I've camped and hiked in Yellowstone without a permit, and Sequoia NP too I suppose. I was far away from any trails or popular spots, and not doing anything stupid, so I didn't worry about it. I figured the worst that would happen is I'd be asked to move along, but I never was. I was never bothered about it all. I talked to a few rangers while car camping like that, but they never seemed too worried about me.