For example, if I owned a sign making business, there's all sorts of people and groups I might refuse to make certain-designed signs for. Even if every one of those groups and persons were legal and the signs would be lawful.
The way I run my business is to have fairly strict rules, which I can selectively interpret as needed to weed out people I dont wish to deal with, or wave for those that I do.
MSNBC was asking rhetorically, to a defender of religious liberty, if a Muslim merchant could refuse service to a woman who was not covering her head. I don’t recall the answer, probably a “Pence-ive” evasion. I would say yes. All over the place, you see signs denying service to those not dressed appropriately (No shirt, no shoes, no service, or even requiring formal dress)
A merchant should also have the right to restrict service based on behavior or the intended use of the product, like poison for suicide, or decorations for a Nazi event.