“If ISIS is not a signatory to the Geneva Convention, how does that tie the US to those rules in dealing with them?”
Because America is a signatory. ISIS isn’t a state either, nor are they uniformed.
America still cannot target women and children who are not committing acts of war against America.
Not true:
The Conventions apply to a signatory nation even if the opposing nation is not a signatory, but only if the opposing nation “accepts and applies the provisions” of the Conventions.[12]
https://en.m.wikipedia.org/wiki/Geneva_Conventions