So the distinction likely comes down to what is required, services which are necessary to living a normal life should be guaranteed to be available. Things like food, shelter, water, electricity must be offered for everyone, even if they are the most detestable people to ever live. For the cake, no such necessity existed. The shop owner indeed offered to sell a wedding cake, but did not wish to personalize it as it was against his beliefs. From this perspective, there is little contradiction. It is not a necessity and is thus not to be compelled. Take the contrived example of a painter who is deadly afraid of spiders. Is it their responsibility to paint a live tarantula? What about a vegetarian cook, must they cook meat? Do you impose a Muslim to drink a toast at an event? None are necessities and each person is entitled to their freedom to decline.