The shop is arguing that, given the artistry involved in creating a custom wedding cake, compelling it to create a cake for a ceremony it morally disapproves of would violate its rights to freedom of speech and expression.
[A]s the libertarian legal scholars Dale Carpenter and Eugene Volokh argue in their persuasive amicus brief on behalf of Colorado, if Masterpiece's free speech claim were accepted by the Court, it would 'apply to a vast range of conduct.' Many of the activities employees and their supervisors engage in on the job have elements as potentially 'expressive' as baking a cake, and this is true of both tiny storefronts and Fortune 500 corporations.
In other words, if something that clearly is art is recognized as art, "public accommodations" laws are going to come apart at the seams and everyone's going to just start associating or not associating as they damn well please rather than buckling down and doing as they're told by their betters.
I don't agree with Lemieux on whether or not that outcome is desirable, but I do hope he's right about the stakes.