Regardless of the legal foundation, there isn't much point in having an ICC without the power to go after leaders of non-party states. You can't rely on heads of state to volunteer themselves for prosecution.
But if a nation decides not to submit to the ICC, then it's up to the UN Security Council to go after them, and the Security Council was set up intentionally to only allow punishment of leaders where every major power can agree. The Security Council at least tries to work with the reality of international relations, whereas the ICC in this case is way out of line.