I just shot a show this weekend, I do only movement and candid shots during the groups (mainly because it is impossible to keep track of who is handling what dog in the breed rings, so with groups I only have one of each breed to contact). I always of course, contact the people putting on the show first and most of them have no problem with me doing that and I always pay a vendors fee.
I NEVER shoot anything an official photographer is. I never shoot any win shots at all, and I of course never shoot anyone by any photographers set up. I only do in the ring movement and candid shots. I have been surprised at how popular this is, people love having their dogs shot in action. These are indoor shows and I am using the 200 1.8 lens so no flash, so it's something that most people are not able to get with their cameras. I often have people come up to me to ask if I am a "commercial photographer" when their dog is in the ring and ask for a card.
On the other side of the fence, for instance, I want to go to a horse show in August to take photos for reference for my art. I wrote to the show committee to ask if this would be OK, (as I always do). Then I wrote to the official photographer and told her what I would like to do and why, and asked her if she wouldn't mind. The committee said if it was OK with the photographer, it was OK with them. The photographer was very nice and said sure and to come up and say hi while I was there. So I can now go without any worries that anyone will be miffed at me taking photos. (also will have model releases if I get some shots I may want to paint. Easier to do at a horse show than a dog show).
I don't have any problems with people takig photos of friends and giving them out but I agree, no one should be selling out from under you. I have not seen that happen personally but I am sure it does!!