The parallel between Christianity and Islam is false. Short version: If Islam insists on inserting its religion into secular life [church & state] then it is open to criticism, parody, satire, etc.
Ed Morrisey of Captain's Quarters blog says it best:
"Editorial cartoons exist to challenge political thought and expose hypocrisy. Among religions, Islam should be the least protected from this form of speech, as it insists on involving itself in temporal political matters wherever it is practiced. Indeed, it insists on dictating political and legal matters, usually in the most extreme terms, and it uses the life of Mohammed as its claim on political and legal supremacy...That insistence on dictating terms of temporal power makes criticism, by cartoonists or editorialists, absolutely necessary in order to combat the stultifying reach of sharia. Islam sets the terms of debate. It cannot insist on temporal rule based on Mohammed and the Qu'ran and then expect people to refrain from criticizing either one...Islam wants to impose its tenets on us, and if we give up the option of political criticism, we have moved more than halfway towards surrender to the Islamists. For those individuals who cross the line into unnecessary offense, the option to use free debate to argue the point will remain open as long as we defend free speech."
Very well said.