What a conventiant investigation of war crimes in Syria

It seems that the UN investigation found evidence of war crimes by rebel groups but only by ISIS:  "The panel also said that the Islamic State of Iraq and Syria, a group linked to Al Qaeda, had summarily executed prisoners, including civilians and combatants of rival rebel groups." How convenient.  Why not say that all those groups in Syria which are not in the same trench with the US there have committed war crimes and that rebel groups in the Saudi-US camp can't possibly commit war crimes ever.