Retrieving an object or going to get help are examples of tasks that might need to be done off leash. However, in my opinion, a leash with no loop at the end should always be on the dog unless the handler can not hold the leash. I said no loop on the end because the dogs foot can get caught in the loop if they are running. The leash is there for control but dropped if the task required it. If a dog is going to work off leash it must be bound by voice control or hand signals. Service dogs aren't perfect but you're right, the handler shouldn't have to repeat commands all the time.

Next time you see them I would ask the person how the does your dog help you or what tasks is the dog trained to do. Usually SD handlers are pretty receptive if you recognize that the dog is there to mitigate a disability but never ask what the disability is. If the person stumbles then you have your answer. There are a lot of "professional" dog trainers who either fake service dogs to get access to different training areas or the dog may be in training and the person isn't following the law.