"I'm sorry, I know I'm not suppose to pet service dogs," they say as they're already petting my dog.
The fact that people know they shouldn't distract a working dog, but do it anyway and openly state it aloud, is irritating. Many service dog handlers are okay with people petting their dog if they aren't too drained or in the middle of an episode. For instance, something that is extremely aggravating is when the handler is having a panic attack, hyperventilating, or have just had a seizure, and people are snarling and calling us rude for not answering their questions or not letting their child pet our dogs.
If someone with asthma were having an asthma attack, would you ask to take a photo with their inhaler? If a diabetic is injecting insulin, would you ask to touch their needle? Obviously, you would not, so why is it different for a MEDICAL-related dog? Working dogs are just as useful as medication, inhalers, heart rate monitors and even glucose meters because they can detect a human's issue before it occurs or before the human notices themselves. (migraines, panic attacks, narcolepsy, seizures, low blood sugar, etc.)
Now, I am not saying that you can't pet service dogs -- but I am saying that you should always ask first and be understanding if the handler says no.