The USSR was mostly hands-off with the DPRK, when the DPRK formed it was more of a merging of the various socialist parties with the remnants of the PRK that were not declared illegal in the North. Further, the US is extractionary towards the ROK, while the PRC is not towards the DPRK, the economic relations are different because the modes of production are different. Further, the scale of US millitary presense in the ROK is far beyond typical for its allies.

As for the source on the US being in charge, here's the Wikipedia article on the ROK/US Combined Forces Command. 1 four star US general in command with 1 four star ROK general as deputy commander. It only applies "in wartime."