You know, that's a dang good question. I don't know what part of the world you live in, maybe the States?
But I live in Romania, and most busyness owners just don't care much about customer services, as long as the clientèle keeps coming back and their profits don't suffer. And that's the situation everywhere, not just one or two stores, restaurants and so on.
As to the reason why, yeah, favoritism could be one. Naturally, Romanian reasons aren't the same as reasons abroad. Here it's just bad manners, weak national identity and social stagnation, and reporting it falls on deaf ears.
But, any busyness owner or supervisor who doesn't care how their colleagues are treating customers either have similar morals, are engaged in nepotism, or are focused on income charts alone, without considering customer services.
What can I tell you, all those reasons are vague, why does it happen at all? You should report it though, every case you come across, never let it go otherwise things never change.