Probably the opposing vew to this is that if you keep your employees happy first, then they will be happy and keep your customers happy. If you have unhappy employees, they might not give a shit about customers and piss the customers off.

We've all been to those places where the employees don't give a fuck about customers, and basically tell you with their body language and tone of voice to fuck off.

Here's an example: https://www.youtube.com/watch?v=N7lGqmZprx0