This is not entirely correct; the US is not a party to the Rome Statute, true, but it in fact Bill Clinton did sign the Rome Statute, but it was never submitted for ratification, and George W. Bush's administration sent a note purporting to retract the signature (as a means of avoid obligations that attach to signatories to treaties even prior to ratification under the Vienna Convention on Treaties, specifically, the obligation not to work to undermine the treaty's purpose.)

The US has engaged more positively (but without any moves toward ratification or even retracting the purported retraction of its signature) with the ICC under the Obama Administration.

You don't have to be signatory to the ICC for torture to be illegal. The ICC is fairly recent. We didn't need the ICC at Nuremberg. We don't need it now. It can be prosecuted within the US justice system, or a new tibunal can be created for it.

It requires voting. and being informed. voting for publicly funded limited campaigns. and being active in the courts