I have noticed myself some badly behaved dogs wearing a therapy dog vest, just a few weeks ago a bloke had four cockapoo therapy dogs in the beer garden of a local pub and they barked at each other the whole time but I've seen plenty that behave well. I think issues arise when there's no minimum behaviour standards to be classed as a therapy dog.
I don't think it's a case of people pretending their pets are therapy dogs, and just whacking a vest on the family pet but that because they're purchased with the intention of improving mental health and giving emotional support to a specific family member rather than being for the whole family, to that person it is a therapy dog, because it gives them a reason to live and purpose.
If a dog is what people need to live a normal life then they should be able to to talk those dogs to the places they need, but.I do think there needs to be minimum standards of behaviour in public for therapy dogs to be allowed the same places service and other assistance dogs are.
There's a few social media accounts where people with guide dogs or assistance dogs document their encounters with poorly behaved therapy/emotional support dogs, things like them running up and growling at a working guide dog, or barking at other service dogs and that's not ok.
Maybe something similar to an agility tests where the dog has to show they are capable of not barking at and approaching other dogs when wearing the vest, no leash pulling, no approaching or jumping at people.