That logic doesn't make any sense, since you can have other hard projectiles, such as a saxophone or a textbook during those times. They'll have to tell everyone to put away ALL objects, which they don't.
A musical instrument would have to be stowed under the seat. A book, though, represents a pretty big loophole. I've never been asked to put a book away during takeoff or landing, even a textbook that's heavy enough to serve as a murder weapon.
There are also way more laptops and tablets around on planes than big heavy textbooks. People tend to bring smaller softcovers when they travel for obvious reasons.
