Most service dogs are trained to stay in work mode until they receive a command from their handler. However, being touched or petted by a stranger can be a huge distraction to them, and any disturbance can prevent service dogs from tending to their owners.
That's why you should avoid communicating with them in any form, either by touching, praising, petting, or acknowledging them—just don't do it!
Source: Guide Dogs of America, 2021. // Paws of War, 2021.
Photo by Unknown.