I could be wrong, but I think the answer to your question as to why, is because it is apparently a known successful way of simply adding 50% power reliably to the stock controller. I actually bought a spare controller and shunts to do this, but decided that I wanted range more than power, so didn't go ahead with it, but as I understand it, it works, and reliably.
EDIT: I think the confusion may be from, I'm assuming OP is asking about a BBSHD controller. I realized after my post that someone not familiar with the BBSHD mod might think it's generally not a good idea, but known effective mod for BBSHD.