How about this.
Have you noticed these signs at your local Costco?
Has anyone heard a statement from Costco as to why they made this decision?
Does anyone know if the statements about Costco's founders are true?
How about this: There are nuts and incompetents out there who have concealed carry permits. Forbidding every concealed carry permit holder from carrying will eliminate some of those nuts and/or incompetents from carrying in the store. Store management believes that by posting the signs, they are doing their part to eliminate nuts with guns in the store, and by extension, eliminating some of the liability they might incur by lowering the number of guns in the store, or nuts with guns in the store, or at least giving the appearance that they did their part to try to reduce the number of both.
Store management and possibly their insurance company is aware of the idea that people with guns in the store may be able to save the day and avert a mass homicide in the store some day, but feel that they have more civil liability by allowing people with guns in the store than they do in forbidding people with guns to enter the store.
If you ever get an answer, let me know how close I came. Who cares WHY they made that decision? It's their decision to make. If you don't feel good about shopping at Costco without your gun, ask around to find a store that welcomes gun carriers. If all the pro-gun people boycotted the big box stores, maybe some of the smaller ones would still be around.