It never meant what people think it means. It means “the customer” in the sense of "the people.” The customer as a whole, not on an individual level.

It means if you have a hat store and you sell red hats, and all of a sudden red hats are unpopular (for some reason) but purple hats become in style, *stock some goddamn purple hats.

It does not mean, and never has meant: “since I am a customer what I say goes no matter what, your hats are now buy one get one free and bend over so I can stick my fist in your butt.”