Starbucks has simply made their policy on firearms to mirror that of state law for whichever state they are operating in. In most states the open carry of a firearm is not prohibited by statute, therefore is lawful behavior.
If you don't feel comfortable seeing a patron of Starbucks open carrying a firearm in Starbucks, by all means stop patronizing Starbucks but you're missing the bigger picture; You should really consider moving to a state that prohibits the open carry of a firearm such as TX, OK, AR, IL, NY, FL, SC, or the District of Columbia. In these states (and district) you can feel confident that the open carry of a firearm is prohibited. However, the concealed carry of a firearm in many of those states is not. In fact, TX, OK, AR, FL, and SC are big concealed-carry states. Your only recourse is to move to NY IL or D.C. where you are obviously much safer. Just look to NYC or Chicago laws. They have some of the most strict firearms laws in the country and clearly they must be the safest places, right?
Megan, you say you'd feel uncomfortable drinking a coffee near a man wearing a firearm. I wonder if you'd be more comfortable walking the streets of Chicago at midnight alone, knowing that no one in Chicago is allowed to own a handgun at all, much less carry one.
I'm not at all concerned about someone sipping a cappuccino and reading a paper who happens to have a pistol in a holster. They're probably the one person in the coffee shop most likely to hold the door for me and call me sir.