I once saw a dog left outside in cold weather while his owner was in the bar tossing drinks back. That made me very mad. In the case of someone leaving their dog out in the rain, well obviously that's not an ideal route. You don't know the specifics of the case though. Maybe the dog is fine in the rain but rips the house to shreds in complete panic mode when left in the dry indoors. Or not, but depending on the dog and the amount of rain, it might be preferable. Unless you wait around to ask the owner you won't know either.
Now I find people who approach dogs that are left tied up to be idiots. Don't go up to a dog you don't know when the owner isn't around and the dog could potentially be stressed. It irritates me almost as much as when parents allow their kids to rush a strange dog to pet it without asking the owner first or giving the owner time to react.