[ghshephard]

As someone else has noted, the set of things that won't harm you, aren't necessarily the same set of things that you are allowed to carry on a plane (leatherman), or check (LiOn batteries).

Also - while you can be trusted to make judgements about your own safety, the airlines have to make somewhat more conservative decisions about the safety of everyone else on the plane.

And carrying packages for someone else is just one of likely hundreds (thousands?) of factors that they are concerned about.

This doesn't guarantee airplane safety (Nothing ever will), but it reduces the number of easy attacks one can make, such that it's becoming somewhat more difficult for a passenger to bring down the plane they are flying on now. Of course, now we have to worry about the pilots (a thankfully rather rare vector)

[chrismcb]

The airlines don't care if you are carrying a package for someone else. Although if a stranger asks you to carry a package, then it is suspicious. But these questions don't really do anything to reduce the attack surface.