The government shouldn't compel speech, but one interesting implication of these sorts of rulings: by supporting the idea that the design of a cake is the personal expression of the baker, it could mean that the baker is liable in the case of a defamatory cake.Alas, that's not our legal environment.
So the question is to what extent government compels service to whom.
Compelled speech...should I be forced to bake a cake which says...
"Atheists will rot in hell"?
I don't know where the line will eventually be drawn.
This ruling suggests that a made-to-order cake, made as per the directions of the customer, is still the expression of the cake maker. That might make it hard for a baker to argue that they shouldn't be liable for libelous writing on a cake, even if they were just following the customer's directions.