If no one else will say it:
This is media spin for constructing popular opinion. Rightly, I'd add, though I agree that it's not technically correct.
Taking a weapon and killing people is in one way not a cowardly act: the murderer (in these cases) will clearly be apprehended and their life will be over (through prison or death sentence). The public interest is served by not framing this as any-form-of brave. That would provide praise to the murderer(s).
The only way that "cowardly" can rightly apply is if we consider the murderers had other ways to forward their cause (e.g. politics). Instead of conducting a peaceful revolution, they 'take the easy path', which is 'cowardly'.
I have a personal hunch that the usage around around 2001, when media needed a way to condemn suicide bombers for attacks on the WTC. Many suicide 'missions' are performed out of a concept of 'honor'. Publicly-labeling these people as cowards is (perhaps?) the strongest response to in swaying the opinions of those that would 'try next.' This being a more effective response than to use any term that would convey power - even that the murderer(s) had the power to be brutal and savage.