Let me try to clarify.
What you may have seen on a guide-dog-in-training or a puppy-in-training is something called a "gentle leader".
a gentle leader doesn't restrict eating or playing (picture source)The gentle leader is used on a few of SAGA's dogs-in-training to teach them not to pull on their lead, and it is only used by trainers who know how to use it and very occasionally by a puppy-walker, but only after very careful instruction on its correct use and only when absolutely necessary. It is only used in cases where SAGA's usual training methods have failed to teach the dog not to pull.
If you're a horsey person, perhaps I can clarify by explaining that the gentle leader performs very much the same function that a martingale does on a horse. A martingale prevents a horse from lifting its head too much and ignoring its riders instructions. Depending the horse and what performance is needed, a different martingale is used- but also not on all horses. It doesn't restrict the horse's natural movement and it doesn't hurt.
a running martingale (picture source)In the same way, the gentle leader prevents the dog from pulling on the lead and not paying attention to his or her handler. It doesn't hurt the dog and it doesn't prevent the dog from eating or even playing. The gentle leader device is used exclusively for training, and when the dog is not "working" the gentle leader is removed.
Once the dog or pup-in-training stops pulling on the lead, the gentle leader is no longer used and it is never used on a working guide-dog or service dog.
No guide-dog or service dog from SAGA will ever require a muzzle. Any dog-in-training or puppy-in-training that shows signs of aggression will not be graduated to work as a guide-dog or a service-dog.
Guide-dogs and service dogs are not guard dogs so you will always be able to approach one without fear - after asking permission from the dogs' handler of course.