You may disagree, but the Supreme Court in Costco v Grill has already supported business owners by coming down solidly on the side of a business, Costco, against a woman (Ms Grill) who would not cooperate with the store manager when he attempted to verify that her dog was, indeed, a Service Dog. I have already posted the steps that a business may legally take to verify whether a fraud is being committed by someone who is claiming that their dog is a Service Dog when, in fact, it does not appear either by its behavior or the handler's behavior in managing the dog to be a Service Dog, as defined by the ADA. You are welcome to look it up. If a business does not choose to take this legal recourse then it is being part of the problem and furthering the cowardly fiction that somehow a business is powerless to impose its policies in such cases.
As for your disinclination to believe the fact of the predominance of owner-trainers that I took the time to share with the group that's your prerogative. Are you suggesting, perhaps, that the thousands of dogs who meet the ADA definition of Service Dogs were trained by the few training schools that exist? Impossible.
As for the driving analogy, the point was the lack of certification of parents who attempt to train their children to drive being as ubiquitous as the lack of certification of service dog trainers, which one poster called for. Of course, untalented drivers are far more dangerous to the public as a whole so before anyone calls for certification of service dog trainers I would expect certification of parents who train their children to drive.