You know why? Because Muslims aren't fire, they're people. And fire isn't a sentient entity, it is a force of nature bereft of choice or cognition of any kind. Just as water seeks its own level, fire burns what it can burn. Muslims have free will. If they choose to riot, that's not the same thing as igniting a fire.
Indeed, the point is proven by the simple fact that the vast majority of Muslims don't riot. More than 17 million people live in greater Cairo. A tiny fraction of a fraction of that number stormed the U.S. Embassy to "protest" that stupid video. And yet, the logic seems to be that the prime authors of Muslim violence are non-Muslims who express their opinions, often thousands of miles away.
I absolutely agree that our devotion to free speech can cause headaches and challenges. But so can any number of non-negotiable facts of life. Anyone with a child knows that having a kid creates all sorts of problems and inconveniences. But few decent parents respond to those problems and inconveniences by loving their kid any less. And as a general rule, only evil, incomprehensibly stupid or selfish people would consider getting rid of their kid to avoid the inconvenience.
There's nothing wrong with exercising sound judgment, even caution, when it comes to offending anybody's most cherished beliefs. But the First Amendment isn't the problem here, the dysfunctions and inadequacies of the Arab and Muslim world are.
James Burnham famously said that when there is no alternative there is no problem. If free speech in America causes a comparative handful of zealots to want to murder Americans, the correct response is to protect Americans from those zealots (something the Obama administration abjectly failed to do in Libya) and relentlessly seek the punishment of anyone who succeeds. Because, as far as America is concerned, there is no alternative to the First Amendment.