Well, as you point out, it’s not a crime if it’s legal.
I don’t get what China gets from propping them up. If South Korea promised to boot the US troops, not to put its army north of the 48 and to give trade benefits for China wouldn’t that be a good inducement?
South Korea would need a lot of Chinese help and $$$ to make the north able to integrate with the South. It won’t be easy or cheap but a unified Korea is good for everyone.
China would have less to worry about militarily and would pick up a better trading partner.
A Unified Korea would not need a bigger military, would not need US troops based there (maybe a naval base in the far south for old times sake). It would become a bit poorer for a while as it rebuilt the north, but the cheap labor in the north being put to productive use would be beneficial in the longer run.
It might take a decade afterwards before the north can be fully integrated, even then it will probably seem like a different country for a while.