Why do jihadists seeking to be martyrs hide their faces w/ a bacalava/face mask?

I got the cowardice part and agree in a sense however, I believe it goes a little deeper as well.

They are hiding because they do not want to give the visible cues through facial expressions/body language to better leverage positions in which to inflict maximum casualties....and in many cases they will have an opportunity to use the device however, many times it will be controlled remotely as well.

I'm sure they have some justification, but I still say they're cowards. But it's just the same as that thing that was going around during the whole "Occupy" movement where alleged veterans/active duty were posting pictures of themselves in uniform but covering their faces. COWARDS.