Ok, it was probably not in violation of the law unless you made a contract for that specific site. Usually you make a contract for a space with certain perimeters. In your case you were not damaged in anyway so they complied with the agreement you made. Blast them if you will on reviews but I think you should learn that when traveling without a toad to leave the campsite so it clear it is occupied. Orange cones etc are often used from what I observe and perhaps keep the office informed. Totally empty sites are vulnerable.