It's also pretty damn difficult to get your hands on a light jet, and then load it up with a huge bomb without it being noticed.
One can make the same argument for chartered transport jets. You can lease a 747 if you want to. That is a much bigger security issue. But the 9/11 lot didn't do that; they went to the much greater trouble of hijacking them.
Ground all transport jets?
Elsewhere, it has been suggested that the "light jet" danger which is all over today's front pages is coming out of the airlines who are getting a lot of competition from light jets. You can get say 5 company execs 1000nm for slightly over £10k (return) which compares awfully well with flying them conventionally, and that is before one costs in the huge hassle of using an airline. The light jet business must be very seriously hurting the 1st class ticket business.