That's also great. Just remember that if I park my car in your public lot, Your property ends where the bottom of my tires start. But that's ok. I won't be shopping at your store.
Look, I'm all for guns. I have several nice ones myself. However, the principle basis of a right to keep and bear arms is the more fundamental right to property. A store, especially a membership store like Costco, is not really "open to the public" as they have a the right to exclude you for any reason--and they also have a right to make rules (shirts and shoes is a common example...not usually required on public property, but typically required by stores (except at the beach)).
One doesn't have to be "anti-gun" to exclude gun-carrying customers....it's probably just a business decision. Since 99% of their customers won't be carrying anyway--the managers of Costco probably think (rightly) that 99% might be more comfortable knowing others won't be secretly carrying guns. Would their being safer true? Probably not...but to a business owner, logical or not, they want to please their customers, so count on them pleasing the 99%, not the concealed carry folk.
I would be appalled if GOVERNMENT mandated no concealed carry in stores, just like I'm appalled when government mandates no smoking in restaurants--but when a private enterprise makes rules of their own--in their private property--well, that's what America and our freedom is all about.
You're free to shop wherever you want--at whatever shop wants you.