To the comment about fleece something or other for the snake to crawl into... I don't think that would work at all. Snakes don't produce much heat on their own at all, and blankets only keep you warm by keeping the heat you put off from escaping. So if the snake isn't putting off heat, the fleece wouldn't be keeping anything in.
I would say that's a bit different... Probably not IDEAL for the snake, but nowhere as bad as wearing one to a music festival or something. At the shows, the vendors probably have areas with appropriate temps available if they are necessary, and probably have a much better idea of when the animal is getting stressed and needs to be put away. Also, most of the people at those expos are mindful of the snakes and are acting in a pretty subdued way. Yeah, there are a lot of people around, but they aren't acting totally crazy and generally know how to handle themselves around live reptiles.
It's not really a benefit to the individual animal, but snakes at reptile shows are often used for educational purposes to teach people who may not have kept a species before about that animal. You could argue that that's good for the species as a whole and only minimally stressful to the snake being shown.