if a dog has felas, the second water touches that dog, all those fleas retreat and hide, the eastiest place for the to hide are areas on the face (nose, mouth, ears, etc). before i even water down a dog with fleas, i soak an area around the collar area, so any fleas not already on the face cant go anywhere near it (most fleas dont hang out on the face, but will only go there after water is used). and to say to never use any shampoo on the face of a dog is really interesting, as that area needs to be cleaned somehow. professional groomer should know how to shampoo a dogs face without shampoo getting the eyes.
As far as flea dip, i would never ever flea dip a dog, and flea dip can actually be lethal to cats. i would absolutely be furious with any establishment using that on a dog without owner permission.
like others have stated, many places make you sign releases, stating that a dog with fleas gets treated then and there. i have worked in places like this, and no customer ever complained. none of these places used dip, only natural flea shampoo. it is pointless to bath a dog and leave fleas on it, not only still infesting the dog, but jumping on every other dog int he shop, and contaminating the whole area.
i do agree with giving a dog a flea bath without notifying the owner (given a release has been signed), but do not agree with flea dip in any circumstances.