There is actually an AKC rule prohibiting unentered dogs being at the show site. It is rarely enforced, because unless someone points it out to the AKC rep, it is hard to know who is entered, and who isn't. I believe the reason is, that when you enter a dog, you used to have to sign a paper copy, and still have a paragraph on the online copy that says you verify with the entry that your dog is up to date on vaccines, healthy and free of parasites. of course, that is all legal crud, and someone could easily lie, or not know about health issues.
Personally, I don't really care if someone brings an unentered dog, as long as it is behaving properly. Lots of non-show people don't realize that we don't let our dogs socialize much prior to, and after a show…they are used to venues where dogs are encouraged to socialize on lead, so they let their dogs come right up to yours. It happens, no biggie. If we are going to a show where we spend the night, we always bring more than one dog, and will often let the companion dog walk around the show grounds after we are done showing...but not usually ringside...just too much going on...