It's not UPS policy only....
The following legalese has some extra phrases which seem to derail some folks:
(a) No person shall knowingly deliver or cause to be delivered to
any common or contract carrier for transportation or shipment in
interstate or foreign commerce to any person other than a licensed
importer, licensed manufacturer, licensed dealer, or licensed collector,
any package or other container in which there is any firearm or
ammunition without written notice to the carrier that such firearm or
ammunition is being transported or shipped
Here is what it's saying regarding the shipper:
No person (you) shall knowingly deliver to any contract carrier (UPS, Fedex, etc) any package or container in which there is any firearm or ammunition without written notice to the carrier
Or, in human:
You not telling them that there's a gun or some ammo in the box equals you committing a federal crime.