Not only do they blow up planes, they're dirty and smelly too.

Either we live in a society in which it is OK to break out the crude, degrading ethnic stereotypes, or we don't. Just like a smoker is never going to be able to kick the habit if he keeps making excuses for why he needs just one more cigarette, we're never going to kick the racism habit if there's a secret stash of vile caricatures we'll only dip into for really, really bad people.
Link.
Now, if the cartoonist is drawing Nasrella and decided to give him a really big turban, no foul. It's the same as giving Bush really big ears. However, terrorists do not go about trying to blow up planes while wearing anything that can be described as "garb". To depict them as doing so is nonsensical unless one is making a broad swipe at Islam in general. Note also the charming suggestions of flies buzzing about the "terrorist's" beard, tying in with the speech bubble's suggestion of incredulity that a muslim is carrying shampoo. Classy.