I think people are confusing different meanings of the word "service".
If someone walks into your store where you sell cakes to the public, you cannot refuse them service unless it's for some reason that you refuse ALL people service (e.g. shirts must be worn, behave in a decent manner).
Hiring someone to provide a service is entirely different. How you are contracting them to do something specifically for you and to your specific requirements and then the person has the right to refuse to provide that service if the requirements violate their beliefs.
It's. Really. Quite. Simple.