Dapperdan is not part of Southwest province. It is part of North Province. The North Province was one of the first 3 provinces defined and had these borders as long as any province in Fanatika. Dapperdan was founded in North Province, it fits there geographically, its history is there. There is no reason to move it simply because a brand new governor in a brand new province wants to govern more cities.
In any case, moving a city to another province simply to favor one governor over another is totally against the spirit of this game. If goonie had wanted Dapperdan so badly he could have simply chosen to govern the North Province instead of contesting to get MAG.