Buried right at the bottom of the article: "the US is not a party to the international tribunal".
Now, you can say they should be. But they're not. So if you look at it from a purely impartial and dispassionate standpoint, the court shouldn't get any special treatment - and the US would be entitled to take such actions against any other foreign group that set itself up and issued threats. It's like me setting up my own private court and then coming to arrest you.
The fact that most other countries are part of it may make a moral case for the US to join but not a legal one, which means that, again looking dispassionately, the US is entitled to do this.