My guess is it's more due to insurance at the venue. I don't know who pays in those situations, but I would imagine they require "no guns" posted and announced. And if there is any form of gunshot injury they have very strong teeth to dodge the claim.
The truth is that you can carry concealed at the NRA convention. It's a myth that they don't allow guns there.
The Secret Service disallows normal people to have guns in the same room where their protectees are speaking. If you want to watch one of the speeches and stay armed, there are other conference rooms / ballrooms with big monitors set up. Many attendees take advantage of this offer.
