I think Starbucks was within their rights to ban guns because of the way that their conceal carry rule was exploited. There were pictures circulating of people brandishing their large semi automatic guns and shotguns and pointing and laughing at them in their photos. This would make many other customers feel uncomfortable, and I feel, for good reason.
Starbucks has the right to ban customers from carrying guns into their stores, even in states with conceal and carry laws. Companies also have the responsibility to keep their customers as safe as can reasonably be expected. Limiting the opportunity for people to bring in weapons can be one step in that effort.