I have a Sams club membership and I do occasionally buy the $1.50 hot dog or slice of pizza, which mimics Costco.
As to showing your membership card at the door, I have never once shown it. They have asked but I have walked right past them, pointed to something off in the distance, and just kept walking. They sometimes ask for your card, they sometimes don't. I don't care to whip my wallet out and waste time so I flat out refuse to show my card.
And yes, people will drive past several restaurants to go to Sams or Costco cafe. You can get a hot dog AND a drink for $1.50. That's a cheap lunch. A McDonalds lunch will set you back $10 these days...