I agree with the author that this is a contract law case. It also is a free exercise and free speech case, but an artist/craftsman should not be forced into an art/craft direction in which he has no interest.
Should I be allowed to sue a carpenter who isn’t interested in building me a plastic quonset hut?
Should grandma moses have been forced to paint a rocketship in watercolors for a scifi fan?
I think it is a 13th Amendment issue. If they sell general pre-decorated cakes, then they can be required to sell them to anyone who asks. However if the cake decorator does not wish to decorate a cake with some message they find repulsive, then forcing them by law to decorate that cake is involuntary servitude. Decorating a cake is art. You cannot compel an artist to violate his own conscience to make art that he finds repulsive.
The same goes for florists, photographers, preachers.