Because people are sometimes stupid in their demands - and there are sites dedicated to recording these instances - the customer is not technically always right. Which is fine, because this quote is generally misused.
It SHOULD be that the customer has the right to make a complaint, where as a sensible reason to do so (like being ripped off, say) is rewarded with proper action (namely, not being ripped off anymore). So, when the customer IS right, the customer is always right. Fight for your rights responsibly, ladies and gentlemen.