The purpose of curfews is not to limit looting. Looting is illegal with or without a curfew in place. The purpose of curfews is to limit peaceful protest. The preemptive use of curfews has escalated this situation, endangered the public, and is almost certainly unconstitutional.
No curfew actually frees cops up if they have to stop looting. But they reallly need huge reform. The way they have been initiating violence during this time is absurd and disgusting. They are there to PROTECT people, not initiate violence.