The photography prohibition is all about legal privacy and property rights. In order to commercially use a photo of an object, it is accepted that you get the permission of the property owner. This is not strictly necessary under copyright law, but it is accepted convention. There is another issue with regard to copyright. Anything made by human hands is subject to copyright. That is images of a copyrighted object may not be published without the permission and, most likely, compensation of the copyright holder. Under current US copyright law, the creator of an object automatically holds the copyright on that object for his/her lifetime plus 70 years. For most antique firearms, certainly most flintlocks, they have long since entered the public domain, but their owners jealously guard access to these items and because of that, you must have their written permission to use photographs of them. Since most people either don't know or care about copyright and privacy law, gun shows just prohibit all photography. You can always ask permission to photograph a gun outside the show venue.
I make part of my living by doing commercial photography, and I must get signed releases for each piece of property, real or personal, that I shoot and releases from each person that I shoot such that they would be recognizable. This is usually all done before the first frame is snapped. Now, after you take a photo, that photo is copyright to the photographer unless they are working "for hire". You can't reproduce that photo without permission of the photographer, but the photographer can't give you that permission unless him/her has gotten the proper releases from the property owners or individuals pictured. There can be a long chain of paperwork required to legally use a photo. It is all governed by some type of contract, which is what a release is. There must be some form of compensation to a property owner of individual for them to legally enter into a contract to give uou permission to use their image. Sometimes the compensation is a $1, sometimes it is copies of the photograph, sometimes it is a lot of money if the image of the object warrants that. The bottom line is that the law recognizes that images of human works have value and the creator or owner must give permission and be compensated in some way. Otherwise, taking and using the image is the same as stealing.
Just so you know, contacts for photography are all about copyright and who can do what with the photographs and when. Most photography contracts are actually licenses that give the client the right to use the photographs in a certain way for a certain period of time. It is common for the client to have an exclusive use of the photos for a period of 2 years for the iniital purpose. After that, the photographer is free to re-license the photos. Also most photography contracts give the photographer the right to use the images from day one for their promotional purposes. However, for big jobs, contracts are carefully negotiated such that the photographer has certain specific rights and the client as certain specific rights. If someone; photographer, client, or licensee; wants to use images for something outside the license agreement, then a new agreement must be negotiated for the new use with some additional compensation.
Hopefully, all of that answers your question.
