The beginning of the movie had an emergency broadcast on TV abt the government informing them of the purge about to transpire. It told them about the roots it came from with the new founding fathers and how it came to be law. The origin of the purge is in the law..
But the purge itself is almost anarchy. It's encouraged, guided anarchy. The government encouraged people to do whatever evil they need to to get it out of their system, so it's not total anarchy.