I don't see a problem with people who bring their service dogs everywhere they go....
If they need those service dogs to help them, then let them take them, I don't think its right for someone to say why or how come?....It's printed very clearly in the ADA laws and they have a right to bring their service dogs with them period....
Second, if someone has a handicapped sticker or sign on their mirror, then that's their business, I dont believe it someone's place to ask such a question of why.....As long they have a sticker to prove they are allow to park on the handicapped spot then it shouldn't matter what you see or think of that person....
I don't even ask such a question when it's none of my business....