Because the one isn't necessarily related to the other? Of course, most items are not a danger for the plane itself but a danger to the other passengers on the plane. It's a confined space. You can't just move away if someone loses his or her nerves and runs amok. Someone running amok with a knife can certainly overpowered, but probably not before several people are hurt.
HTB.
Oh boy: person runs amok on the plane, splashing folks with water from his bottle and getting shampoo on them. Plane goes down in flames. Riiiight.
