"War crimes" are infractions against International Humanitarian Law, most notably, and infractions against a package of pther laws commonly grouped as "International Law of Armed Conflict"

The letter is demanding sanctions in response to violations of International Law. That is a totally different level of laws which, in the context of the current conflict, America and Israel and breaking equally.

I also highly doubt that America is committing fewer violations of IHL than Israel, but I've got a middling confidence interval on that.