Except it isn't, as anybody who's worked in a position where they have to deal directly with customers knows. What it is is a business slogan, a mantra; simplified version of a policy of appeasement that encourages staff to make the customer feel as though they're always right. It's a policy that entails that staff should always treat the customers complaints as valid and do everything within their power and responsibility to deal with the customers issues, but almost every business will also tell it's employees to never state that the customer is right unless it's objectively provable, because customers are not all delicate flowers and beautiful garden and as a company's first responsibility is to their employees they cannot allow for customers to take the piss and strong arm employees based on the presumption that because they paid for it their criticism is inherently valid.
'The customer is always right' doesn't mean that the customer is always rights, it means 'make the customer think they're right to avoid issues'. When the customer says something that is demonstrably false, for example 'Stellaris is a sham', then the policy simply requires that the employees should not tell the customer that they're talking out of their arse. It doesn't mean you have to accept their stupid statement as though it were carved in stone atop a mountain.