Quick side note: you are within your rights to refuse service based on political affiliation full stop -- it's not protected under the equal protections clause.
That being said, the issue is not about denying service full-stop, but the right to refuse expression of values you find to be wrong. Believe it or not, these cases are important for everyone and guarantees that the state can't force you to create messaging in support of (i.e. endorse, which is a form of speech) something you disagree with.
It's not granting the right to discriminate. It's protecting your first amendment right to not be compelled to engage in speech you disagree with.
For example, say I go to a bakery run by devout Muslims and request a cake that depicts a cross with the phrase "only through Jesus may you find eternal life" underneath. That baker may be uncomfortable with the idea of creating that design as it not only goes against their own sincerely held beliefs, but may conflict with some negative views they may hold of Christians or Jesus (or even the particular denomination of the customer).
That Muslim baker has every right to refuse the design of the cake on free speech grounds. Religion is a protected class in the equal protections clause, so the Christian may feel like they're being discriminated against, but it's the message (which is considered to be speech) and not the individual being a Christian causing the issue.
That Muslim baker cannot blanket-refuse any Christians from buying any cakes. If that Christian customer instead asks for a blank cake that they'll decorate themselves, the baker must sell it to them or else they are violating the equal protections clause. In that case, service is being refused based on the traits of the customer rather than on the particular message being expressed on the cake.
It's silly and I think people would be better off just accepting the work and taking the money. If I was aware of a business that made cakes, websites, whatever -- but refused certain designs based on their personal views, I would simply discontinue any further support of them. I'd prefer a business who puts their own shit aside and serves whomever wants to pay them.. but to compel them to suck it up and either compromise on their views or close up shop is directly contradictory to one of the most important rights we recognize here -- to speak freely and without cohersion from the state.
The business owner isn't doing anything wrong with their signs, but they're completely missing the point of the decision and comes off as a bit silly.
