Treating all customers as morons is certainly a bad idea - but so is treating all customers (including those who are morons and/or assholes) as kings, very often.

It's a tradeoff between the asshole customer's money being just as good as the pleasant customer's, and the effort you have to expend to get it.

A customer who repeatedly ties up service people for hours with baseless, aggressive complaints about their $20 purchase is simply not a customer worth having.