The customer is always right

Me: Could I please get some ID for this cash back?
Customer: makes angry face, acts agitates, sends ID
Me: Who is "D"?
Wife: Me
Me: OK, I'm gonna need you're ID to cash this check since it is made out to you.
C: She doesn't have it!
Me: OK, I'll send it back then for "B" to sign since you are using his ID.
C: Why do I have to sign it?!
Me: Because "D" doesn't have her ID and the check is made out to her. You can just sign the back of it as well so you can cash it.
C: Well, just send the whole *** back and we'll go inside!
Wife: NO!
(pause)
Me: What would you like me to do?
Wife: Just cash the check!
Me: I will need to...
Wife: HE DOES NOT NEED TO SIGN IT!!!
(pause)

I turn the speaker off, turn to my co-worker, and say, "They are giving me such a hard time? What is the big deal about signing the check?"
My co-worker says, "Well, since I know them you can just go ahead and do it for them. But you are in the right, what you are doing."

Why is it so hard for some people? Why do they not realise that asking for ID is for their own protection?