Imagine if "shopping day preference" was added to anti-discrimination law. That's innocuous, right? I mean, who would want to discriminate against someone just because of their shopping day preference? And then the following Sunday, Andrew, who fancies himself a Sunday Shopper, strolls down Main Street, visiting store after store, as is his habit, when he comes across a door that won't open and a sign that says "Closed". Furious, Andrew calls his lawyer and a lawsuit is filed claiming discrimination against Andrew on the basis of shopping day preference. Andrew, you see, claims to be a Sunday Shopper.
The store-owner is confused. She has nothing against Andrew. She'd be happy to serve him on Monday, or Tuesday, or Wednesday, or Thursday, or Friday, or Saturday. But Sunday, for her, is different. Her religion calls it the Sabbath, and she doesn't want to work on that day. She's not trying to discriminate against Andrew. If he wants to shop on Sunday, then that's his business; but she doesn't want to be involved, not on a Sunday. She wants the freedom to live her life as she sees fit, and to not be involved with activities—such as shopping on Sunday—that she considers to be wrong.