Per Benediti:

The chance of such a terror act on a private aircraft is way lower. 

I remember a stretch when I could not take to the air in my humble little Beechcraft: 9-11-2001 through 10-01-2001.  Then, private planes could resume flight but FAA regs about flight plans somehow got just a little bit stiffer.  I can't figure out why.  Maybe it was because something weird happened in NYC and DC.

Don't sell short the terrorist potential of a privately-owned aircraft.  One could do all sorts of nastiness with an ultralight if he were so disposed.  FAA has taken what I consider appropriate safeguards (pardon me if I defer to go into specifics) but nothing can render the skies safe if a lunatic wants to abuse the beauty of flight.

It is purely a disciplinary matter.  That is why we have prisons: very secure places where even the rottenest cuss cannot get his grubby mitts on anything that is potentially dangerous.