The USA has signed all 4 geneva conventions and all 3 amendment protocols, but not ratified 2 of the latter. Source: http://en.wikipedia.org/wiki/List_of_parties_to_the_Geneva_C...

And the conventions quite definitely do apply to some degree when a signatory fights an opponent that is not a nation (and which therefore, by definition, cannot be a signatory). The details are a bit difficult, especially when a conflict is both internations and involves non-nation parties, but it's definitely not correct to say "our opponents are not signatories, so all rules are off".