Somehow, I expect that France would not tolerate Monaco shelling Cannes, nor would the English tolerate the Welsh shelling Bath. The US wouldn't write off San Diego or Del Rio. So, why should Israel not respond when the Arabs of Gaza either tolerate or actively help Hamas to attack Israeli cities?
The Israelis should be praised for their restraint. Most reasonable people (including Europe through its entire history) would have just wiped out the entire population and let the refugees find another home. Israel has at least attempted to not kill civilians.