There are many reasons not to join "that Court." Anyway, our Constitutional provisions are superior to treat obligations, so we would be out of synch with many of the provisions of law of other jurisdictions.
For example, freedom of speech guaranteed by our Constittution is relegated to inferior status under European doctrine, and certain speech is crininalized there that is not here.
And why not adopt sharia law provisions if we pick and choose. Being a homosexual, or insulting Allah or Muhammed, are a status or acts punishable by death in some jurisidictions. If we want to subject ourselves to"other law," a reduction in our freedoms, why "The Hague?"
So called international law also, for example, limits the way pirates are fought, providing that a proactive assault on pirates is not permissible, nor are raids on their bases. So we'd be required to behave as emasculated victims emulating the Europeans, fettered by nonsensical "rules." Certainly, we wouldn't want to commit "the crime" of interdicting pirates without being limited by "The Hague" rules.
Why would we want to hamper our self-defense activities to conform to the mandates of some people, who have demonstrated repeatedly that they are quite inept in many areas?
Why would we accept their ban on capital punishment?
What about conduct toward refugees?
Our values and their values are not the same. It would require gross modification of our ways to subject ourselves to their mandates. And why would we think that they have developed a response in any way superior to ours, or in any way better for us, if operative.
What is the motivation for joining "the Hague?" Who is proving what to whom and for what purpose?
Finally, what is the incentive of joing "The Hague?" What benefit does it confer on us as a nation. Does it impair or facilitate our ability to respond?
The ultimate question is "What's in it for us?" And none of this "feel good" nonsense. If you want to feel good, we'll take up a collection for you and buy you a vibrating chair. I won't contribute, but some may.