As I stated, they do NOT post signs in the Nashua; Springfield MA; Waltham, MA; Dedham, MA; Avon, MA Costco's! The signs would carry NO WEIGHT under MGLs or NH RSAs either.
If I'm carrying, I'm carrying in their stores and everyone else's too.
If you write Stop & Shop, Shaw's, Big Y, etc. supermarkets and ask them their policy on guns in their stores, I can guarantee you that you'll get a similar answer. What then "on general principal"? Where you going to shop?
Rules are written by lawyers and they are going to CYA for their client, to the point of ridiculousness.
As for the remark
I don't know what stores you've been in, but I can vouch for 90% of the Costco folks that work the doors in Avon, Waltham and Nashua . . . they are all US Citizens and speak English-American! I know many of them by name and they know who I am, as I chat with them on my way in/out each time. I learned long ago to be nice to the lower level folks, and it pays off. I did that in college and had the privilege of parking in the Faculty lot as an undergrad for all 5 years at NU! It cost me a few Christmas gifts each year to the parking lot attendants and a friendly "good morning, how are you" each day on my way to classes. . . a very small price to pay indeed!
YMMV