Why do cops hassle people who open carry?

Most often, at least from my reading, because of idiots like this manager. 

A guy shopping in a Wal-Mart with his fiance, with their buggy half full, gets hassled because of PSH.

I usually don't carry at WalMart, but for some odd reason I decided to stick my Glock in the holster tonight before I went out.
I had been in WalMart maybe 15-20 minutes, and my cart was half-full already (It was going to be a quick trip) and a twitchy little squirrely fellow with an Assistant Manager tag on his ID comes up to me and asks how I'm doing (I wasn't even thinking about the gun, and just figured he was coming through the aisle and was being friendly).
I realized why he was there with his next question, however. I'm going to break this down into the stereotypical conversation format for my ease.
Manager: Are you a cop?
Me: Er, no?
Manager: I see you have a gun.
Me: Yep. I usually carry.
Manager: Do you have a permit for that?
Me: No, here in PA it's actually not required that you have a permit for open carrying.
Manager: Oh, okay. I was just checking.
Me: That's alright, I make it a point to inform anyone who's unaware
And then he scurried off at about "make it a point", when I was hoping to have another minute of his time, maybe answer a few more of his questions, calm him down, etc.
So naturally, I figured he was just busy and thought nothing of it.
A few aisles later, my fiancee and I were discussing the relative merits of one bag of Lay's potato chips over another (She went with Kettle Cooked Meyer Lemon & Rosemary, if anyone cares. I'll be glad to do a full taste write up ) and I happened to glance at the entrance, as we were in one of the main circuit aisles going from the entrance to the back of the store, and I see two large fellows standing there dressed exactly the same, with the little manager almost throwing himself to them in fear.

It looks like this went about as well as it could under the circumstances. Kudos to all for keeping their head, excepting the idiot manager with the heavy load in the seat of his pants.

Are we too frightened of "scaring Whitey" to open carry? We have National Ammo Day, and National Buy a Gun Day. Do we need a National Open Carry Day or two to help some of these jerks to get over their hoplophobia?