Are you sure you read the article.....
They were staying out of it, as they have always allowed (and still do allow) people to bring guns into their stores in states that allow it. They did not willingly get involved in this mess.
"Pro-gun activists have used our stores as a political stage for media events misleadingly called “Starbucks Appreciation Days” that disingenuously portray Starbucks as a champion of “open carry.” To be clear: we do not want these events in our stores. Some anti-gun activists have also played a role in ratcheting up the rhetoric and friction, including soliciting and confronting our customers and partners."
They have every right to tell people they can't bring guns onto their property, but that's not what they're doing. They have not banned weapons, they simply made their position clear so that pro or anti gun people can't use them to further their own agenda. They clearly failed.