Why I don't buy the 'customer is always right' line anymore

I read a post last week from a server bragging about how they let a rude table walk all over them just to keep a tip, and honestly it rubbed me wrong. I've worked the register at a diner in Akron for 6 years, and the moment I started treating every complaint like gold was the moment I got walked on daily. Respect has to go both ways, that's not a hot take, that's just basic survival behind a counter. We had a guy last Tuesday scream about cold coffee he'd left sitting for 20 minutes, and my manager still comped his meal. Has anyone else drawn a line with abusive customers and actually seen management back them up, or is it just me?