Texas Star, the officer you saw worked at a plant, or hub. He just got dispatched to check on the complaint rather than send a USPS Inspector. I have met a few of those officers in the past, the last one was eating a Blimpie sandwich, and talking with his mouth full.
As far as toting a Swiss Army knife into a post office, you may do anything you are willing to suffer the consequences for, LOL!! A customer entering the postal facility for business purposes is of course, subject to the law, but we do not have metal detectors nor do we search people. Out of sight, out of mind.
As USPS employees, we have the need for knives to remove plastic wrap and straps from magazines and heat shrink from pallets of parcels. We actually have small knives similar to paring knives and ring knives (rings with hooked blades) for such purposes. I personally carry a Black Hawk switchblade in my pocket, and have for years.