Let's look at federal law.
Under 18 USC 922(g)(1), it's unlawful for anyone:
(1) who has been convicted in any court of, a crime punishable by imprisonment for a term exceeding one year;
to possess a gun or ammunition.
Under other subdivisions of 18 USC 922(g), it's also unlawful for a variety of other persons to possess a gun or ammunition, including fugitives, unlawful users of controlled substances (which would include anyone who uses marijuana lawfully under a state law), persons dishonorably discharged from the Armed Forces, aliens present in the U. S. on other than an immigrant visa (subject to a few exceptions), and a few others.
When I reviewed some of the applicable case law a while ago, I found cases in which federal courts of appeals had affirmed convictions for being a prohibited person in possession of a gun even when the defendant didn't actually have the gun on his person or in his hands. So, for example, it was sufficient to support a conviction for the defendant to have been living in a house in which there were unsecured guns or ammunition over which he could exert dominion and control.
I didn't see any cases in which a conviction for a violation of 18 USC 922(g) was based on a person merely being temporarily in a place where guns were present. On the other hand, at guns shows there are typically guns sitting out on vendors' tables where anyone can pick them up and handle them. And based on case law, a prohibited person even handling a gun is a violation of 18 USC 922(g).
So under federal law, it seems to be an open question whether or not mere presence of a prohibited person at a gun show is a violation, handling a gun at a gun show would be. And there is also a question of whether a convicted felon's presence at a gun show violates the terms of his probation or parole.
Furthermore, if the operator of a gun show knew or had reasonable cause to believe that a patron were a prohibited person, the operator would be well advised to eject that person. There is reason to believe, based on case law, that knowingly permitting a prohibited person access to guns could support an aiding or abetting charge.
4thPointOfContact said:
...I'm not even going to get into the whole 'touching is not possessing' aspect.
Are you contending that it is not? Do you have any legal authority to support that contention? In fact, under applicable federal case law, you're wrong insofar as "toughing" includes handling or being able to handle.