The customer IS NOT always right contrary to old folklore.
THIS part, I absolutely agree with.
I knew a guy....He used to live in a tee pee. Among his many faults, he would cheat any and every business that he could. He is the guy that would have a tool for years, wear it out, then buy a new one from the store and then stuff the old one in the box to return it as defective.
Self serve junkyards out here would use a special marker to mark parts that are available to return for refunds or exchanges. He went to a craft store to find paint that matched so he could return worn or damaged car parts that he never actually bought.
He was the type that also defended shoplifters based on the bullshit opinion that if they needed something and didn't have the money, they were entitled to it. You know, like the idiot looters that steal TVs during riots so they can feed their families...
The customer is not always right, that is a fact. Some are assholes, some are dishonest, some have some delusional impression that a business exists only to screw the customer so they are fair game to be mistreated.
If the OPs story about this "Dave" guy was the first and only time that I've heard it, I'd think nothing of it but come on, guys....There are numerous instances of similar treatment from the man.