My thoughts are simple.
1. Their store, their rules. Although open to the public they are not public places (long ago established by the courts). They are private properties owned and operated by a corporation. They have the right to set policy and enforce it under threat of criminal trespass. No different than your home or place of business. Anyone who doesn't like it can shop elsewhere. I believe the greater right is of a property owner to give anyone the boot, provided it's not a discriminatory practice. There is no law prohibiting discrimination against gun owners. You also have no right to freedom of speech there, or of freedom to peacefully assemble, to campaign for office, etc.
2. None of these companies care about letters regarding firearms policies. While there may be strength in numbers they certainly don't care if groups of gun owners periodically threaten to boycott their business. It's just not enough loss of business to change the policy of a large, franchised company.