It's private property that is also commercial property.
Costco has every right to ban guns on their property.
And people have the right to tell Costco that they will not give them business if they ban guns on their property. And also to encourage others to boycott Costco over this issue.
Your position is more like that of the Dixie Chicks - who want to exercise their rights but still demand that patrons subsidize them if the patrons disagree with their stated positions.
That doesn't make any sense to me. My position is that Costco has every right to ban weapons (or smoking) in their stores, it's their private property. Shoppers who disagree have every right to not shop there, and to make it known to others why.
Maybe you got Sir Francis viewpoint confused with mine.