China rules Tibet because for most of it's recent history China has been militarily and economically dominant enough to rule Tibet.
There's noreal right or wrong about it. It is what it is.
Why does the United States rule California?
Why does Egypt rule the Sinai?
Why does England rule Wales?
Why does Australia rule Tasmania?
There is no perfect world where everyone who wants their own nationality is free and independent. It doesn't and can't exist because there are too many contradictions.