I like the berets, but where has it been said that wearing them is mandatory? I've just checked nanu SOP for uniform regs
USCM and USCM-affiliated personnel are required to be wearing appropriate clothing during operations. Deploying marines and MP'sshould be wearing their uniform, boots, and armor at a minimum. Other crew must wear their uniform and assigned footwear. Headwear, if any is worn, must be your properly assigned headgear or one of the standard-issue USCM headgear.
That reads to me that MP's can discard the beret if they want but can't wear any non approved headgear.
Am I missing something?