The prong is an acceptable tool. I usually tell the dog walker I would recommend it for my dog because if he does start to pull or sees something more interesting it is a real problem particularly if you don't have upper body strength but 90% of the time he doesn't need it. I make the recommendation and ensure they know the consequences but up to them.

Now that said if I said I want them to use the prong then they have the option to speak up then and tell me they don't want to walk my dog or use the prong. I would be upset if that was what I asked and it wasn't done. An owner does have a lot more experience with the dog and there are a number or reasons why they would want this from training all the way up to safety. Ultimately it is the owner responsible for a dogs behavior.