It is an interesting question which action can be called 'cowardly attack'. We have exactly the same problem in German. As soon as someone (often: a politician) calls a terrorist action 'feige' (cowardly) someone comes up pointing out that a terrorist who is sure to die during his attack can be called nearly any name but not a 'coward'.

See, e.g., McGrath making that point two years ago here when a suicide bomber was called 'coward' by a Mudcatter. The use of 'coward(ly)' in the criticised sense, however, is widespread as you can see easily by looking at the many 9/11 threads here (they are linked in one thread which makes the study easy). Both British and American Mudcatters have used the word in that sense.

I think (though I am not sure at all for my knowledge in etymology is quite restricted) that to call the WTC attack an action of cowards uses the older and first sense of the word coward (not all dictionaries mention that sense). The word comes from an old Norse word and the original sense is slightly better preserved in the verb 'to cower'. As far as I know the old word just meant to squat, e.g. behind something. Now, fear is one reason to cower (in the old sense), another is not to be seen when preparing an attack (which itself may involve a big personal risk and even result in sure death).

A 'cowardly attack' in the old sense is just an attack that comes as a complete surprise for the attacked, an attack that comes from someone who went to great lengths not to be spotted before attacking. Such attackers are often not cowards in the newer sense, but cowards in the older sense of attacking unsuspecting victims in a surprise attack.

Wolfgang