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Every country has an interest in ships being safe from arbitrary seizure (or missile strikes).

If that were the case, I can think of numerous other initiatives that would have raised an outcry from them. Instead, Russia and China have both been on the arbitrary-seizure-and-strike support group when it suits them. Which is not wholly unusual in international politics - it only means that when a polity objects after otherwise being quiet on or supportive of a type of action, one should look as to why that is.

That being said, the reasons 'why' are mostly a matter of academic curiosity or political strategy for third parties, considering the US strikes are unjustifiable morally and legally.

Yeah, but the individual interests are important as it is China and Russia that attack other ships if and when it deems them appropriate. It's not only the US.