I'll just suggest that the US population is now way more sick than in years past. I'll also suggest that dogs are now trained for more tasks than ever before when dogs were just for the blind. I'll also say Federal Law does not absolutely require vest, ID, or flashing neon sign.
Now -- There are two questions business owners and employees can ask people that come into their businesses with dogs.
1. Is that a service dog?
2. What task is the dog trained to do?
That's it. You cannot ask why a person needs a service dog or what their disability is. You cannot ask them if the dog is certified as a service dog or require them to show a certificate or other paperwork (that's because there is no official certification for service dogs, and because anyone can buy a certificate for their dog on the internet even if they aren't disabled and their dog has no training whatsoever, so showing you a certificate of some kind would be completely meaningless). You cannot require that they have their dog demonstrate the tasks it can perform for them.
Asking anything other than those two questions above is a violation of a person's rights under the Americans with Disability Act. If you or your employees violate the ADA, you may find your business faced with hefty fines. As stated in: What Questions Can Business Owners Ask People with Service Dogs? - Yahoo Voices - voices.yahoo.com
dated Dec. 2013.
Do I think in the instances you cited people are working the system? Well, very likely, or maybe not.
I hope I didn't sound arguing or anything -- I just thought I would answer since no one had.