Argument: The US cannot help reform the ICC unless it joins
"U.S. Policy Regarding the International Criminal Court". Congressional Research Service. 29 Aug. 2006 - The Assembly of States Parties adopted procedural rules for its activities at its first conference, including rules setting forth the role of observers and other participants.82 Observers are entitled to participate in the deliberations of the Assembly and any subsidiary bodies that might be established. Observer States will receive notifications of all meetings and records of Assembly proceedings on the same basis as States Parties. They will not, however, be permitted to suggest items for the agenda or to make motions during debate, such as points of order or motions for adjournment. Thus, the United States may be able to participate substantially in Assembly debates as well as proffer and respond to proposals, even if it never becomes a party to the Statute.83 The United States may also use its position at the United Nations to communicate to the Assembly of States Parties.
As noted, the United States is not able to vote in these bodies so long as it does not ratify the Rome Statute. It may not nominate U.S. nationals to serve as judges or cast a vote in elections for judges or the Prosecutor (or for their removal), or vote on the ICC’s budget. It will not be able to vote on the definition of the crime of aggression or its inclusion within the jurisdiction of the ICC, when the matter is considered at first Review Conference, or on any other amendment to the Rome Statute, unless it ratifies the Rome Statute.