The way the law is written...anyone pretty much can walk into a store with a "service dog." There are very exact questions the store is allowed to ask you about the dog and if they ask the wrong question it could be considered discrimination according to the ADA and you can scream bloody murder. I know some people with disabilities and service dogs carry around an ADA handbook and will show a questioning employee their rights. At the end of the day, if you make a big enough squack and threaten legal action they'll probably leave you alone and let you do what ever you want (as long as you have a well behaved dog). If you are falsly claiming your dog as a service dog, and the dog does something bad, you'll be just as liable for injuries if your dog was a service dog.
SFGSSD is correct...the only way to "close" the loophole is mandatory certification. But that will never happen, mostly because it will be a huge obstacle for handler trained SDs and also for the huge variety of service dogs and disabilities there are. Plus...I really don't want my tax dollars going towards a service dog certification program...the current system works, while its abused by some, it probably helps a lot more than it gets abused.