If the considerations mentioned by Millenium
were the problem, why doesn't the US participate with the organization of the court and try to change its rules to something it likes better, instead of total opposition in the form of the American Servicemembers' Protection Act
Furthermore, many countries have judical systems that don't work exactly like the US one. US citizens who travel to such countries and commit a crime there will face a trial under such a system. Does the US prohibit its citizens to travel to such countries? Does it use military might to protect its criminals from justice? No. In fact, they are usually extradited to the country in question.
I mean, the USA also expects soldiers from countries that consider the death penalty to be unacceptable to hand over Osama bin Laden to US forces, should they happen to find him. So on on what grounds do they justify demanding special protection for their own citizens when they commit similar crimes?
Additionally, according to the UN Assiciation of the USA, the ICC does provide a presumption of innocence and, more importantly, will only try someone if the country they live in refuses to try them. If this is correct, the excuse would be total bullshit.