As has already been mentioned above, we have stayed in Rest Areas for some shut eye (not camping!) that have signs up stating "no overnight camping". We have asked LEO's and security guards many times if we are OK to do so, and whether in Wyoming, BC, or elsewhere have been told the signs are there to enable them to be able to move undesirables along. We were never disturbed and always given the thumbs up to stay overnight irrespective of signage.
Less paperwork and involvement for them, than dealing with the carnage of an over tired driver a few miles further down the highway from enforcing based on signage.