Since I don't hear and read the news reports you do.. I don't know who is calling them 'cowards'.
But the reason you would call "them" cowards ... is because we don't know WHO... they are.
Certainly nobody meant to call the terrorists who went to their own deaths cowards... penetrating the country and starting the act requires balls... but going to your own death by purposely crashing a gigantic aircraft at 550 mph goes BEYOND balls.
The act takes bravery.
Doing it without taking credit is the cowardice.
But it is of course the only smart way to fight against a hugely bigger foe.