It's a good point regardless, though, and I don't envy the position of trying to draw the line between what's protected speech and what's not.

Edit: it seems the cake was not requesting custom writing -- custom writing was explicitly protected as free speech in another case. It seems in the analogy of the burger joint, that case was more akin to a burger joint which only sold "I endorse your decisions" buns. Which makes me as glad as ever that I'm not in charge of being the arbiter of what's speech and what's not.