If they wanted to commit a terrorist act they could use them to restrain someone else or to strangle someone. Or, as someone on another thread posted (the poster said she is a stewardess), the metal parts of the seat belt extensions could really do some damage if you hit someone with them.
Thanks for your responses. I didn't realize that the extensions were useful as a weapon. Seemed to me that a belt with a large buckle would accomplish the same thing, yet those are not banned. Oh, well.