Rant: Why Does Costco Require a Membership Check at the Door?

I waltzed into Costco today only to be asked before entering the heavenly store to show my Costco card. Is this some trandition I’m not aware of? I usually smile at the “greeter” when I walk in, but when they demand to see my card, I always question why. What’s the point of showing my card? If I don’t have a card, can I not look around to see if I want a membership? Are the products that Costco offers so exclusive that only people bearing the precious Costco membership card can see them? The checkers pretty much do a background check on you to verify your identity before letting you buy anything. So why do I have to show the front of a plastic card to shop at Costco? Sometimes there’s a wait to get in the store due to employee insistant to see as many Costco logos as his or her eyes would allow in one shift.
I love nearly everything else about Costco. So save me the five seconds and let me spend money.