Firstly...screw Walmart. I have had horrible experiences with every one I've lived near and tried to shop in regularly.
And as others said, this just adds to my hatred for them. This is extremely disrespectful.
Also, I have a friend who became disabled. She has a Rottweiler that, after becoming disabled and healing from that inceident as well as she would get, she trained to be her personal service dog. She trained it to get tissues for her when she sneezed, grab the cordless phone if she couldn't get to it (which she nicely equipped with a sort of handle, to prevent doggy drool in her ear!),basically, do tasks that she can no longer do easily at all. The dog is not only good for things like those hard daily tasks she has to go through, but also a confidence builder. Without him, she would probably be lost. She did not want to get a service dog, she wanter her dog to be her service dog, and she worked hard through her disability, with the constant help of friends, to train her boy to do these things.
Now, there are people out there, sadly, who mock the service dogs by saying that because their dog has a neat party trick, it can be a service dog. However, there are also people who have become disabled, and have trained their own pets to help them out.
So you really can't discriminate with "what ifs" here...since any dog with a service dog vest, can indeed be a service dog, with the few exceptions of those who are just made to be flashy by their owners, for whatever reason.
And even what seems like a small disability to you, may be hard for the person who has it, so whatever small task that dog is trained to do that you don't think it necessarily a big deal, that person can believe it is the most important thing in the world, and they deserve just as much respect.
Keeping in mind of course, these self-trained service dogs should have the same courtesy in public as those trained by organizations. My friend I spoke of went through extensive socialization classes with her Rottie before she was disabled. He, just like other service dogs, won't take food from strangers, won't walk up to strangers, etc etc.