On its website, the NRA explains:
Note: North Carolina State law prohibits the carrying of firearms in the Charlotte Convention Center, and the Time Warner Cable Arena. In addition, the Rules and Regulations of the Charlotte Convention Center prohibit the carrying of firearms in the Center. Pursuant to Time Warner Cable Arena policy, all individuals entering the Arena will be subject to a magnetometer security check.
An NRA staffer elaborates on the no-guns policy, in a letter posted by the Huffington Post today:
Thank you for contacting us.
The Charlotte Convention Center does not allow the carrying of firearms, both open and concealed.
The large size of our event and the fact that many of the largest convention centers are in some of the most restrictive cities leaves us with relatively few convention centers large enough to accommodate the Annual Meetings. In an effort to provide all NRA members a better opportunity to attend the Annual Meetings, it is important that we move the event around the country as much as possible. While we will not consider bringing the Annual Meetings to a city with gun laws we feel are restrictive, we must however deal with convention centers that have restrictions simply because there are so few convention centers that both allow conceal carry and are large enough to host all of the events that comprise the Annual Meetings.
Thank you for your support!
NRA Member Communications