This is all that is required to post a store or restaurant against the carrying of firearms. While other states may require more explicit or more extensive measures to post a premises, North Carolina does not. Carrying a firearm on a posted premises is a Class 1 misdemeanor in North Carolina.
It seems that not only are the gun prohibitionists trying to encourage owners to post their premises but some of their activists are taking it upon themselves to "unofficially" post the business. Sean Sorrentino at An NC Gun Blog has the full story of a business in Greensboro that was "posted" by one of these activists.
Fortunately, the gun owner who was buying dinner at this establishment questioned why they had "posted" their premises. Not only had the business not done this but by going through their security tapes they were able to identify the anti-gun activist who had done it.
My suggestion if you do see one of these decals on the door of your favorite store or restaurant is to ask to speak to the management. It may turn out that they aren't anti-gun but rather are victims of a hoax perpetrated by gun prohibitionists. If so, I'd suggest contacting Grass Roots North Carolina with the information. Their contact info is here.
And if the business is really posting their premises and is anti-gun, report that information to GRNC's High Risk Restaurant Project.
No Lawyers - Only Guns and Money: BOLO