Adam,
I've never been to the GCSAA (or GIS) Show (though I'd like to), it seems to me that this is by definition a trade show. Unless you're a super or somehow involved in the operations of a golf course, why would you want to buy this kind of equipment?
In the case of the PGA Show, I think at least part of it is that sometimes companies are showing prototype equipment that isn't yet available to the public. And while attendance has been down in recent years, it's still a fairly crowded place. The primary goal of the PGA Show is to introduce product to the media, golf professionals and other industry types, not to sell it right there on the spot. I wouldn't necessarily be opposed to opening the doors to the public, but add the element of selling products there and I could easily see it becoming a zoo. After all, people can't buy the cars at the New York Auto Show, can they?