Over the years the distinction between public and private spaces has become obscured. Which is why Starbucks is finding it so difficult to insist that customers do not carry weapons while in their establishment. It is because over the last several decades a doctrine of public accommodation has developed in the law such that when some area is adjacent to a public sphere — a street or road or park — it no longer enjoys private property rights, the authority to determine what happens there.
It all came about because of the impatience with racially discriminatory merchants and costumers. If they were understood as having firm private property rights, they would have to have their racist practices protected by law, which the courts were unwilling to sanction (unlike the protection of porn!). In particular, in a decision by the United States Supreme Court, handed down invalidating a law enacted by referendum in California pertaining to the right of people to sell their property to whomever they choose, Justice Byron White explained that the California law (Art. I, Sec. 26) enacted via Proposition 14 (in 1964) "authorized private discrimination," even though, he added dubiously, only "encouraging, rather than commanding" it. (Actually it only tolerated it!) He added:
The right to discriminate, including the right to discriminate on racial grounds, was now embodied in the state's basic charter, immune from legislative, executive, or judicial regulation at any level of the state government.
And for him, a loyal modern liberal justice, that was unacceptable! Yet that is exactly what is entailed in the notion of a right — its exercise, wisely or unwisely, is shielded from others' interference. Justice White himself made this evident, albeit disapprovingly, when he observed: "Those practicing racial discrimination need no longer rely solely on their personal choice. They could now invoke express constitutional authority, free from censure or interference of any kind from official source". And what's wrong with that? It's the same with everything else objectionable the constitution protects, such a flag burning.
Notice that by prohibiting racial discrimination as a matter of legal mandate, the court removed the issue from the realm of morality or ethics. How could one freely make a personal choice to discriminate (or not) if government has the legitimate power to stop one from discriminating not as a government official but as a private citizen, within one's private domain? If I want to restrict the potential buyers of my home to only Mormons or White Protestants or Hungarian refugees, that ought to be my business, no one else's. But no, the Supreme Court of the supposedly freest country of the world chose to prohibit bad choices by its citizens. That is exactly like censorship by the government, plain and simple. And recall how so many American commentators were appalled at how Muslims reacted to the Danish cartoons that made fun of Islam! For Muslims what the cartoonists and the papers that published them did was every bit as awful as racial discrimination was to Justice White and his colleagues on the Supreme Court.
Now back to Starbucks and gun rights. Turns out that because the U.S. Supreme Court ruled that the Second Amendment protects individual Americans who want to own and carry firearms, this now means Starbucks isn't free to decide about whether its costumers may do so in its coffee shops. Why? Because these shops are "affected with the public interest," because they are located on streets which are public spheres and because government regulates them. Here are proprietors who want to apply their own, possibly sound standards of safety within the establishment they own and aren't permitted to do so because, well, the property is no longer deemed to be really theirs at all but part of the public sphere (square).
Slowly and surely everything in the country will come under public — that is, government — jurisdiction, treated as if it were a courthouse or some other sphere where public administration goes on. The logic of the slippery slope is inescapable here. Moreover, if public officials make bad decisions, they will drag the entire country down since there will not be any private sphere left where those, like the owners of Starbucks, could institute practices that could well make better sense than what the public officials insist everyone must adopt.
One of the ways a free society deals with dubious practices by private citizens is to protect the liberty of those who find fault with such practices to protest, including by refusing to allow them in their own private spheres such as their places of business. But because this principled approach does not immediately do away with some of the despicable practices of the citizenry, such as racism in commerce, the eager beavers have now thrown the baby out with the bathwater, sacrificed individual liberty for the sake of coerced decency. This is exactly like when others abandon liberty for the sake of security. Plague on them both.