The problem is that there is no document that assures the dogs are qualified.  For that matter there is no one standard that can be tested for, just general guidelines.   

On the other side, every 'guideline' I've read allows a property owner/manager/bus driver to eject a dog that is misbehaving and creating a disruption.

These days there opposition to all regulation.  But regulatory standards can add clarity to what's required.  (which is the concern of those trying to deal with disability guidelines in education and the Trump administration removing those guidelines.)