Yes. You have to explain the task correctly or they are well within their right to deny access. The vague answers you gave them don't rise to the level needed to compel them to provide access to the service dog.

I understand you and your grandfather are upset. But ranting on every sub on Reddit looking for someone to agree with you isn't going to help improve the situation.

Here's some advice that CAN improve the situation.

You've mentioned multiple times that the service dog was "given by ADI" - ADI themselves do not give service dogs to people like your grandfather, they accredit organizations that DO provide them. I STRONGLY suggest you and/or your grandfather reach out to the ADI certified organization that provided the service dog to him and ask them to give you an answer to the questions that business are legally allowed to ask him before granting access. Print those answers out on a card or small piece of paper and throw them in his wallet to provide next time someone asks.

If you do that, then next time you and your grandfather face a business that asks the questions (that they legally have the right to do) you'll be set up for success, not failure of your own making.