You know 911 was quite a long time ago, and what were excusable knee-jerk responses to it at the time are really not so forgivable now. I've been thinking on and off this morning about how a tennis racquet can be considered an unacceptable piece of carry-on luggage.
That's absurd I thought. But then it occurred to me that you could take out the strings and garrotte somebody with them. But of course tennis racquet string is not in fact a banned item (although who knows?!) So it would be less hassle to put your garrotte in your pocket.
So actually I've concluded that this regulation is as spectacularly stupid as I first thought.
(I play quite a lot of tennis - I'll ask round my club if anyone can think of how to hijack an aircraft with a racquet. Feel free to post suggestions below. Unless you've got a really good idea, in which case don't post it below.)