There's a whole range of service animals who help people; some are seeing eye dogs, hearing dogs, or a whole assortment of alert dogs for people suffering from diabetes, epilepsy, and psychiatric problems. There's even support animals for people with autism, which helps manage the things that make everyday life stressful. None of these animals or their owners deserve discrimination in any way what so ever, and they are trained for as long as any person is for a job so they are perfectly suited for any public space. There is, however, an issue of people not wanting to separate from their animals for reasons other than a physical disability or difference and those people are hurting what should be a system to help people who truly need it.
So here is the break down of what makes a service animal a service animal. Does it perform a physical task for you? If yes, that's a service animal and as long as it is well behaved and well groomed you are allowed to take it into public anywhere you like in accordance with the Americans with Disabilities Act. If the dog doesn't perform a physical task for you like seeing, hearing, alerting, or in cases of people with autism helping them feel comfortable or sensing oncoming panic attacks, then it's not a registered or trained service animal and that is just a pet you're bringing into Walmart with you. Don't be fooled by places like big box stores that don't inquire with you about your pet because that's their company policy, it is perfectly legal to ask if your companion is a service animal and what service your animal provides for you. If it's a registered "emotional support dog" or just an animal with a fake vest bought on Amazon, then they aren't legally required to let that animal inside. If you bring your pet into a store and it's badly behaved, dirty, smelling all the food, then that pet should have been left at home and it's ruining things for people who actually need their service animals. Businesses aren't legally allowed to ask for proof the dog is a service animal and are certainly not allowed to ask for proof you're disabled, however if your animal is posing a threat to a business or the costumers within then that business is allowed to exclude that assistance animal from their facility.
There are emotional support animals who are actually very beneficial, such as for people with PTSD or other forms of mental disorders, but if it's not a trained service animal then business owners have a right to turn them away as they are not compliant with the ADA. Taking your pet everywhere you want simply because you don't want to leave them alone is not only abusive to a system made for people with disabilities but puts the safety of actual service animals at risk. If you're just going to the store and will be back home in an hour, leave Buster at home. He will be okay!