The Nazis tried the same argument at the Nuremberg trials. They claimed that they weren't bound by the laws of war (e.g., Hague regulations) since Poland and other states hadn't signed them. The court dismissed the argument and stated that certain rules are binding whether both parties are signatories or not. In Israel's case it is even worse since indiscriminate attacks have been outlawed since basically forever. At the Nuremberg trials, the argument "there is no precedent" had some merit, today it certainly does not.