No matter the weather, it's still not safe to leave a dog unattended outside of a store alone. It would be the same if it was a child. Anyone walking by could take the dog (or child). If you are taking your dog for a walk or a drive and you know you have to go into places that don't allow dogs, then leave the dog at home or run errands later without the dog. Unleashing the dog may not be the nicest approach, but you can't always be nice to people or try to tell them the proper way to take care of their dog cause they don't want to hear it. Unfortunately it takes something tragic happening before people "get it". So better to teach them a lesson with no harm done then to see the animal get hurt for real.

Some dogs may like rain, but it doesn't sound like this dog was having a good time.