There are probably a lot of business that post for some reason (customer relations, insurance...) but really don't care if customers carry. And as discussed elsewhere, even if the owner has banned carry in general, he can allow people to carry on a case by case basis, as long as he is not engaging in illegal discrimination (race, sex, religion,....)
Part of the ordeal I went through is because people get told by a buddy, Uncle Fred, etc. that is what the law requires. This flows from the common practice of not charging somebody for ignoring the notice provided by the sign. It is better for everybody just to ask the person to leave (and hope for compliance). This practice does not mean that the person did not commit a trespass, just that he won't be sanctioned for it.