(edit: The mere fact that Starbuck's has 130,000 barristas at any given point in time makes it quite likely that someday one of those barristas will sue them for discrimination because they couldn't get the shifts they wanted. If the company can prove that the shifts were assigned using an algorithm and not by a person, they have a good chance of winning their case.)
I'm not sure what evidence you're asking for... That people sometimes feel discriminated against?
This is wrong. An action can be ruled discriminatory even if the intent wasn't such. Though if it is considered "vital" to the survival of the business there can be exceptions.
But honestly, companies with only white collar workers face downtime too, but the employees still get paid
It is about making money. Plain and simple. Occum's razor at it's finest.