The reason is that it has not issued any findings yet. It is going to in a few months.
This is clever wordplay, nothing more.
Edit: fuck me, it's as if no one gives a shit about the truth anymore. It's all kneejerk reactions.
This is a non-story. Governments don't just willy nilly declare that somebody has or has not committed a war crime. It goes through an entire process. That process has not completed yet (in the US anyway). There are no findings yet. Therefore until the findings are released, the US has not found that Israel has done anything.