This isn’t about defining a business model. It’s about defining discrimination and protected groups. By your logic above, the photographer could charge a black couple more than a white one. I know that’s not what you mean, bit it would be the potential result of how that law would be interpreted.
At the end of the day, a Trump rally is not a protected group, so a business can say no. Just like a shop proprietor can refuse business to said rally goers, but not to a protected group.