Political affiliation is not protected, religious affiliation is. It's true that the Right has been doing their level best to politicise their religious feelings into public life, so that barring Trump supporters effectively excludes Evangelicals and a majority of Catholics. This may not be their desired outcome, but perhaps they shouldn't have tied their religious sentiment to political causes.

Religions are protected classes under the constitution, political groups are not. Free speech is also protected. The combination of these factors means that weather the shop keeper in OPs photo is breaking the law is entirely dependant on how you interpret the constitution, which is what the supreme court is supposed to do.