I'm a deafblind dog guide user (I'm currently working with my second guide dog) and the same thing has happened to me.
I don't know how many times I've said to people, "You know, there *is* a human being on the other end of this leash."
I've also had people pet my dog without permission. I used to have a sign on my dog's harness which read, "Do not pet me. I am working" but I finally removed it because people constantly ignored the sign. (I've even had sighted friends tell me that people would read the sign, look at me and then pet my dog. Grrr!)
Sometimes I wonder if these people think they can get away with petting my dog because I'm blind. Even after I catch them in the act and request that they stop petting my dog, I've had people say things like, "You're blind. How do you know I'm petting your dog?" at which time I'll reach out, grab their hand or arm and say, "This is how I know."
What they fail to realize is that I can tell my dog is being pet by her reaction both on leash and in harness (it's no different than a sighted person getting to know the behavioral habits of their pet dog), but oftentimes I do not have the time nor the patience to explain this to every person who wishes to pet her.