I manage a big box retail store and the reasons we don't allow dogs are:
1) General liability - We are, in the best case financial scenario , partially legally liable for what happens in our store. If the dog bites someone, knocks a display over onto someone, breaks product, trips someone with their leash, urinates/defecates one the floor and a slip and fall happens, ect, ect we are liable. Basically we don't want to be sued.
2) Some people have allergies.
3) Retail stores are open to all people and everyone entering should be to go about their business in peace, including those children and adults who are scared of dogs.
4) Any dog that comes into the store must be a licenced service dog with tags. No tags means the dog doesn't come into the store, including "comfort" dogs.
I love dogs, have a dog myself, but a store unrealted to pets is no place for animals. Again it's a store open to everyone, it's not like people allergic/fearful/just don't like dogs are going to a dog park and complaining there are dogs there. They avoid that place, dog owners can avoid bring their dogs into the store. Courtesy goes both ways.
Also even when it's summer and it's hot outside, that doesn't magically absolve us from legal liability. When informed your dog isn't allowed to be in store, don't get indignant and say "It's hot, what am I supposed to do with my dog?"
The answer is "Not bring it into the store".
Summer doesn't just sneak up on us one day out of the blue, it literally happens every year. It also gets hot during the summer, this is also not a new thing either. Getting angry, acting like a meteorologist and informing me of the current weather conditions isn't going to change the fact your dog is still not coming into the store. Either figure it or don't, as long as your dog stays outside it's not my or anyone elses problem.