I think they are considered originally Chinese for game balance issues.
If they were originally Chinese, then Russia cannot capture them for income or to give England the NO for an allied controlled orange territory.
Likewise, if Japan puts an IC or AA in these territories, and later loses it to the allies, they become virtually useless. This also prevents the allies from taking it, building there and fighting against the Japanese.
Basically, making them Chinese territories makes life a lot easier on Japan.