It's probably because, well, you can interchange tags lol...if you have rabies on only one dog, but have two dogs, if you bring dog #2 out you could potentially put dog #1's collar and tag on and be "fine". They also need to make sure there is nothing wrong with that particular vaccine-if 100 dogs are vaccinated with Ft. Dodge's rabies vaccine, lot # 10A044, and 70 of those dogs have bit someone-that's the problem! Or if 5 dogs come up with rabies and they have been given that lot # vaccine-well, there is also something wrong with the vaccine. I hope you get why I'm saying...