why do they say the customer is always right when they aren't?

In one of my training days in retail I was told the customer is almost never right but you have to pretend they are. That about sums it up.

sometimes one has to take it in the butt to make a buck. but yeah sometimes i'd love to tell certain customers to eff off when they can be a pain knowing they may never buy again...but there's always the reputation factor

you have to accomodate them if you wanna make some bread

Because you want return customers