For China and Russia, you don't intervene anywhere ever. If you intervene in Syria, why not in Russia? Why not in China? They have questionable policies so they are big believers in sovereignty and want everyone to leave them alone. Creates too many problems for them to intervene while they got shit in their own country that needs to be fixed. (in addition to economic ties).
This is one of those "just sayin'" moments: The US used the same argument of sovereignty when the International Court of Justice ruled that it had violated international law by supporting the Contras and awarded Nicaragua reparations, which the US refused to pay.