It makes no sense. Laptop bags, diaper bags and ladies' purses are OK, but a little flight bag or back pack might be used to transport... what? .
Here's the logic. (Note that I am not saying it is good logic)
All Young Adults carry Backpacks
100% of YP -> CB
Most terrorists are Young Adults
51-100% of T -> YP, 100% of YP -> CB
Therefore
Most terrorists carry backpacks
51 - 100% of T -> CB
Hey! Thats how I can spot terrorists!