Even if this convention was a "gun-free" zone in the normal sense (i.e. a lightly guarded school or shopping mall) gun owners are fairly orderly and law-abiding folk. They generally don't rebel against authority and carry guns into gun-free zones, even it objectively puts them at greater physical risk.
But this does not sound like normal circumstances. The whole problem with gun-free zones is that there is often no one close enough to provide an armed response to a mass shooter. As the saying goes "when seconds count, the cops are minutes away". An event with actual security (presumably Secret Service) there is an entirely different tradeoff.
I don't know the security details for that event, but presumably there are security plans that would assure most gun owners of their safety. Is your plan to provide that level of security to all gun-free zones?
Apparently they were banned because vice president was there and that was the requirement of his security entourage.
So representative of the federal government forbade law abiding citizens from carrying a gun within the premises. Somehow there was no opposition to that.