No one must be allowed to above the law, as this will result in vast abuses of power, as we have seen in the past. If the USA joins the ICC, then there will be greater incentive for decision-makers to carefully consider the true and far-reaching consequences of their actions.
Why is this important?
George W. Bush and his White House committed war crimes, including the greatest war crime of all, starting a war of choice. Yet these people are still free, and one of them (Bolton) is trying his best to repeat his crimes on a grander scale.