That’s right. If this were a can of beans, everyone would have a right to buy it. But it’s a cake with a design and a message . That is the difference. Elton John has the right to refuse to play at Republican events. This cake designer should have the same freedom.
Those two aren’t related - Elton John would be a private decision. The web designer is a business. Completely unrelated.
Since Elton John played Rush’s wedding and allowed his music at Trump rallies I am not sure he thinks in those terms but you make a point.