and why they are ill advised.
It is not just because aversive actions are "out of bounds" for the positive trainer, it is because they lack meaning for the dog.
The German language is a great one for very specialized nouns. Qualzucht is a German word for the purposeful breeding of animals with genetic defects; this has been made illegal by nationwide legislation. While there is a straightforward translation of this word, to truly understand the meaning in context requires that you learn and understand quite a bit of German language, history, current events, and the history and current culture of dog usage and breeding in Germany. Kind of a lot to understand one word.
In traditional dog training, the dog is molded into a sit by pushing down on the rump while lifting the head with leash and collar pressure. Through repetition, the dog learns to release pressure on it's neck by assuming the sit position. When the sit correction is applied (a swift, emphatic jerk straight up on the leash), the dog knows how to escape the pressure. In traditional training, most teaching follows this pattern. The traditionally trained dog is well versed in the application and removal of aversive stimuli.
Positively trained dogs do not understand the application and removal of aversive stimuli. They don't "speak German." Negative reinforcement is not in the vocabulary and syntax they have been taught. It is possible to teach them enough for one specific application to make an impact on a specific behavior, but it's a little like taking a college course in German to understand one word. The return on investment is low.