Walter Russell Mead offers an explanation:
...(W)hen television cameras show the bodies of children killed in an Israeli air raid, Jacksonian Americans are sorry about the loss of life, but it inspires them to hate and loathe Hamas more, rather than to be mad at Israel. They blame the irresponsible dolts who started the war for all the consequences of the war and they admire Israel's strength and its resolve for dealing with the appalling blood lust of the unhinged loons who start a war they can't win, and then cower behind the corpses of the children their foolishness has killed. The whole situation strengthens the widespread American belief that Palestinian hate rather than Israeli intransigence is the fundamental reason for the Middle East impasse, and the television pictures that drive much of the world away from Israel often have the effect of strengthening the bonds between Americans and the Jewish state.
This automatic Jacksonian response to the Middle East situation overlooks some important complexities in the Israeli-Palestinian conflict, and in the past America's Jacksonian instincts have gotten us into trouble. But anyone trying to analyze the politics of the Middle East struggle as they unfold in American debates needs to be aware of the power of these ideas about war in American life.
In any case, when Israel brings the big guns and fast planes against Gaza's popguns and low tech missiles, a great many Americans see nothing but common sense at work. These Americans aren't mad about 'disproportionate' Israeli violence in Gaza because they don't really accept the concept of proportionality in war. They think that if you have jus ad bellum, and rocket strikes from Gaza are definitely that, you get something close to a blank check when it comes to jus in bello.