...The court doesn`t "clear" this murderer of anything... No, you are wrong. The court says, "Dichter could not be sued because he was acting as a government official at the time". The judge didn't say, "we have no authority to trial this case". It means, the US court cleared him of crimes that was accused against him (this is usually called pre-trial execution).
UN: Gaza could be 'uninhabitable' by 2020 if trends continue (AP)
from the article: U.S. District Court in Manhattan clears Dichter in war crimes suit