Note that the keffiyeh comes in a variety of colors and patterns; the key element seems to be that it is worn on one's head (I will confirm this after tracking down one of our local Lebanese).
I'm a little unsure as to the logic chain followed here, but I'm guessing that it goes something like:
- Terrorists wear keffiyehs
- A keffiyeh is a square piece of cloth
- A scarf is a square piece of cloth
- Therefore, anyone wearing a scarf is a terrorist