I think whether or not someone touches your pet should be up to your pet. If he doesn't desire being petted, then by all means scold whomever is chasing him around trying to pet him.
If though he stands still and is enjoying it, where's the harm? Sure the kid may have had ice cream on his hands, but have you ever known a cat to say "thank you, no" to the offer of ice cream?
It might be a little unethical to tell someone your cat is a service cat, but it's unlikely that it will work anyway. If my cat were posing as a seeing-eye cat for example, he would just watch me bounce off of stuff, giggle and mutter "I saw that" and the jig would be up.