Do You Remember Me, Gerald Ford?

Wow, I haven't posted in so long. Moment of Silence
Anyways, moment over.
I wanted to talk about something that has been troubling me for a number of weeks, but first I have to recall a bit from Dean Martin Celebrity Roast I watched when I was a kid.
Rich Little (famed impression-doer, very good) gets up to do his roasting when he decides to impersonate President Gerald Ford for a few laughs. He backs away from the podium, walks back toward it, trips, and says "Hello, do you remember me? If you don't, I have an American Express Card..."
Oh, I have a point. I promise.
So, I'm not actually sure what that whole bit means (other than the tripping bit, which was rather tickling), or why it's particularly funny, and not just plain strange. But I have to say, the other day while I was at work, I thought about this.
Why, you ask? Good Question
I work in retail, and because the computer isn't capable enough to differentiate among debit or credit (damn you technology, I thought you were supposed to think about this stuff so I wouldn't have to!), I am obligated to ask customers every time they pay with a card if it's debit or credit.
So I did that. And, mind you, half the time I can't even see what kind of card they pull out, and I must admit I'm not paying attention to a whole lot besides how hungry I am or why Keanu Reeves has a career as an actor. See, me...*points to her brain*--not so much up here. Anywho, based on all that, you can imagine my utter confusion one day when I politely and sweetly asked "debit or credit?" and the infamous customer said, "It's an American Express Card."
Ok...I know my brain is designed to process answers to questions I didnt actually ask, (such as, "will that be debit or credit?" and not "what kind of card do you currently possess in your hand?") but that still doesn't really help very much. Luckily my manager was around to explain to the customer that there's no way I could have actually known that AmEx only makes credit cards, barring any sort of actual interest in the world of finances. No thanks, mister. In any case, this has happened to me twice already and I only have to ask...
Why do people assume that random young people actually know that certain companies don't make ATM cards? Furthermore, why do they assume we even care? I'm sure it was all explained to me very carefully (some day, sometime....I cant say I was paying much attention....I want pie), but I would really rather someone not use "It's an American Express card" as a response to debit or credit. It's two words people, just PICK ONE!