Iran was under UN sanctions at the time Meng violated those sanctions. China is on the UN security council, and as a permanent member, they have a veto. They did get a vote, and they voted yes.

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.