What happens if Xi Jinping decides that he wants something that you do not want? He will not let you vote if one day his foreign policy goals involve nice Americans.
Are you somehow not aware of this? They were a founding member of the UN, and have been on the Security Council since it's beginning. They have abstained or voted yes on every UN action that involves the SC (international peace and security).
Meng violated additional US laws... which she is only subject to because she was operating in the US. Just as a US citizen is subject to Chinese laws when operating in China. Surely you're not suggesting that foreigners should be allowed to go into another country and violate their laws without consequence.
The PRC became a permanent member of the Security Council (with a veto) only since 1971. Taiwan was the permanent member from 1945-1971.