It will all be legal when US and China are subscribed to the same set of international laws - when will the USA for example, ratify UNCLOS? Obviously there wouldn't be lawyers or courts if everyone agreed on everything, the passage of US coastguard ships through waters that are disputed is seen as illegal by some (me included) and legitimate by others (apparently yourself). I'm not going to argue about the legitimacy of this, I'm only going to say that it's considered by some to be illegal and, until the US sits in the United Nations Convention on the LAw of the Sea to ratify that, then it's illegal here.

Let's agree to differ and leave it at that because I can see this will go nowhere

Cheers