One thing to note, in NC if you charge admission, then you can't carry there. This includes movie theaters, concerts, etc, no matter how small.
So, bottom line, if they had to have the event in NC and you paid to attend, then you couldn't carry anyway.
EDIT TO ADD: Just found this exclusion:
§ 14‑269.3. Carrying weapons into assemblies and establishments where alcoholic beverages are sold and consumed.
(a) It shall be unlawful for any person to carry any gun, rifle, or pistol into any assembly where a fee has been charged for admission thereto, or into any establishment in which alcoholic beverages are sold and consumed. Any person violating the provisions of this section shall be guilty of a Class 1 misdemeanor.
(b) This section shall not apply to the following:
(1) A person exempted from the provisions of G.S. 14‑269;
(2) The owner or lessee of the premises or business establishment;
(3) A person participating in the event, if he is carrying a gun, rifle, or pistol with the permission of the owner, lessee, or person or organization sponsoring the event; and
(4) A person registered or hired as a security guard by the owner, lessee, or person or organization sponsoring the event. (1977, c. 1016, s. 1; 1981, c. 412, s. 4, c. 747, s. 66; 1993, c. 539, s. 165; 1994, Ex. Sess., c. 24, s. 14(c).)
So, they could carry there if the NRA gave them permission. So, maybe they should have looked for somewhere else.
EDIT TO ALSO ADD: It may also be they could not find a venue large enough to accompdate the convention where alcohol was not sold, which makes it off limits