The Purge isn't completely without restrictions.
Certain government buildings and essential infrastructure remain protected.
Government officials and designated essential workers may receive special protection.
Trying to continue a Purge crime after the ending siren is illegal again.
If someone attacks another person at 6:59 AM, they're protected by Purge law.
If they continue at 7:01 AM, it's murder.
That makes the final minutes some of the most chaotic.