My groomer said it is not the best idea to do it, unless the dog has been doing a whole lot of scooting. Something about it being better for them to express them naturally as they poop. I asked her about it because another groomer did it as a matter of course with every grooming, so I sort of expected it. (My dog hadn't been scooting.)
I've never heard anything about having to be licensed to do this, I know that groomers as well as vets and technicians do it. They can even teach you to do at at home yourself.
I have no idea and don't see why but you can check your State laws on line, I'm sure.