China has a long history of disregarding United Nations and US sanctions. During the second Iraq war they defied UN sanctions by selling anti-aircraft defense systems to Iraq directly endangering US and coalition fighters.
Keep in mind that US sanctions against foreign companies doing business with Iran are opposed by:
The United Nations International Court of Justice, which ordered the US to withdraw sanctions, to which the US responded by pulling out of international agreements.
The European Union, which has attempted to block its companies from complying with US sanctions.
With that said, the US is free to sanction anyone for its own interests, that's its right as a sovereign state. It is also its right as a sovereign state to refuse market access and trade to any company that violates its sanctions.
But arresting foreign nationals in other countries for violating US sanctions? That's the equivalent of Russia arresting American business executives and extraditing them to China for violating Chinese sanctions against Taiwan - and yes, there are sanctions against Taiwan, which are regularly ignored by the US, of course. This is a massive escalation and will undoubtedly cause a major international crisis. Stay tuned.
I mean you’re right that the UN has a lot of realpolitik, and you need to take those decisions with a grain of salt. But the rulings you are referring to are an instance where the US took a different ideological stance to the EU as to the best course of action to deal with Iran (which nobody wants to have intercontinental ballistic missiles, but really doesn’t want it to have nuclear weapons).
The example I gave and what it looks like Meng did at Huawei are examples of the Chinese government actively supporting weapons programs in hostile regions. I very much feel that she should stand trial for the deception she did constructing these shell companies to sell US export controlled items to Iran.
Edit: verb tense.
https://books.google.com/books?id=grGMAwAAQBAJ&pg=PT221&lpg=...