They also cover treatment of civilians, medical units, and infrastructure. Amongst other things.
> The Geneva Conventions do not apply to wars with non-state actors or states who are not participants to the treaty
Yes, they do.
The protections are not as extensive, but the conventions also cover any armed conflict within the territory of a contracting party, such as the Syrian Arab Republic. They also cover occupation of a contracting party's territory, whether armed or not.
> The Geneva Conventions would never apply to traitors on your own side, which is the case here.
The conventions don't make any exception for traitors. They certainly don't allow Syria to take an ISIS fighter prisoner and avoid their convention responsibilities by handing him over to the US.