President Obama and Bibi Netanyahu are on the same page when it comes to the justification for Israel's bombardment of Gaza. Netanyahu : "No country in the world would agree to a situation in which its population lives under a constant missile threat." Obama: "There's no country on earth that would tolerate missiles raining down on its citizens from outside its borders."
It's true that if, say, Canada were lobbing missiles into the US, the US wouldn't tolerate it. But here's another thing the US wouldn't tolerate: If Canada imposed a crippling economic blockade, denying America the import of essential goods and hugely restricting American exports. That would be taken as an act of war, and America would if necessary respond with force--by, perhaps, lobbing missiles into Canada.