Service dogs are protected by the ADA (Americans with Disabilities Act) due to the important work they do for their handlers. Because of that protection, service dogs are allowed to go anywhere their handlers go (with a few exceptions), even when in training in. (Each US state has its own laws in regards to in-training service dogs and access, not all allow service dogs in training to have the same public access rights as fully trained service dogs).
Service dogs need to be able to focus on their handlers, as well as take in and evaluate their surroundings. It’s important to understand that service dogs are still dogs. Trying to pet, talk with or otherwise interact with a service dog while they are working can be distracting and hinder the dog from doing their job. If you feel it is truly important to approach the dog and their handler, one recommendation is to speak directly to the handler while not interacting with the dog and ask for permission, while respecting their right to decline greetings. Please keep your own dog away from working dogs, and don’t offer snacks to a service dog on duty.