[eximius]

Race, gender, religion, disability... I'm probably forgetting a few other protected classes that you can't discriminate against.

That said, you only cannot discriminate on the basis of one of those listed above. There are unfortunately many people who will try to discriminate using a rule which indirectly singles out a group (I am intentionally omitting examples, use your imagination).

There are, perhaps more unfortunately, valid reasons to serve one cohort and not another that can have bias along these lines. Once again, the actions of some hateful few can ruin things for the rest of us.