Hiring for availability is 100 percent a thing. I’ve filled out an availability chart at every retail job I ever worked. They don’t ask why you aren’t available and it’s about making sure they can cover all of their positions. Discrimination is when you refuse to hire someone because of their religion, and having a bunch of people that are that religion already on staff is generally a pretty good indicator it’s not because of their religion.

It’s absolutely possible to hire so that someone can call off without causing a chain reaction that kills the schedule and triggers over time. Employers just don’t want to do it.