Yeah, and I think in a lot of instances the prejudice has been earned.
It's incredibly difficult with dog owners. You don't know what you don't know - and it's not hard to find handlers that honestly believe they've got it all under control, but actually have very little understanding of their own dog's behaviours.
But how many times have you heard an owner say "Don't worry, he won't hurt you" when the dog is displaying very clear "I'm gonna kill you if you come any closer" behaviour, right!? This is where people get bit, because they don't know what they don't know when it comes to dog behaviour, what their dog is capable of, and the messages the dog is sending with their behaviour.
Many many many (pet) dog owners overestimate their dog's training/their control over their dog. These will be the ones that will be faking ESAs/SDs to get special privileges.
"trained" is very subjective without:
I see a role for standardised behaviour and handler assessment here. And not necessarily a government regulated testing regime (which is what we have in my State - a standard behaviour test that all Assistance Dogs and handlers must pass to have public access rights). Kennel councils, for example, could have a role here as an alternative to a government-run standard.
And no, it doesn't even have to be necessary to include this under ADA. But for flying....where they propose that you have to sign that your SD/ESA can "behave"....really isn't too much to ask. In my opinion.