I think that the man who spoke to you to discourage your use of the fenced area only did so in the best interest of your dogs. Yes, the shelter brings the dogs up to date on their shots but were the dogs allowed in the area previous to the shots? Parasites (Coccida, Giardia, etc) and other infectious bacterias can be lurking in the pen if not cleaned out regularly. While your dogs may be all up to date, vaccinations can not prevent some things like parasites. Coccidia can be transmitted from one dog to another so fast - within 24 hours - and all a dog has to do is step in a TINY piece of fecal matter. There is also the possibilities of fleas, kennel cough (not likely without other dogs present). Just be aware of the situation. I would ask the shelther owner if they clean the yard out properly on a regular basis and if shelter dogs are permitted inside. Good luck.