Because you can presume that the character had no idea what he/she was doing when they set off the laser.
I don't buy that crap. Your character was just using it to destroy the enemy ship! Occum's Razor: Fire it at Earth, you'll kill a butt load of people!
While he/she may have done it accidentally, it doesn't take rocket scientists to say "Hey, this is a death ray. Hmmm...better not fire it on Earth."
And this American agrees: What do they have against Canadians? They're a good people! Of all the places our character could've zapped, it had to be Ontario, Canada.
Methinks someone of a higher up had a bad experience in Canada and this is their way of saying http://tvtropes.org/pmwiki/pmwiki.php/Main/TakeThat