Use of the term "Cowardly Act" in regards to violence

Ah, but you are to agree that the perpetrators lacked courage, or at least that is what the person making the statement intends when they make it.

They are framing some uses of force (whether one calls a given use force violence or not is a similarly politicised matter) as cowardly while others (generally those that they would themselves sanction, such as military operations they support) as not cowardly, or indeed brave.

Linguistically, it is a valid use of the normal sense of the word cowardly. Whether one agrees with the statement is another matter.

You're not far off in your idea of what is deemed "unmanly", particularly in those cases dealing with assymetric warfare, where one side may portray their enemy as cowardly because they plant bombs and flee rather than engage in open combat and the other side may just as well portray their enemy as cowardly because they are better armed and have air superiority which they may use rather than engage in open combat.

I think the phrase cowardly act is idiomatic.

When this phrase is invoked, it is intended to cast the mismatch between the combat status of the attacker and the targets as dishonorable. In each of the examples cited in the question, though the perpetrator may eventually be apprehended by combatants, the majority of the targets are civilians.

Note that this is different than notions of asymmetric warfare, in which unconventional methods or mismatched conventional methods are employed by combatants chiefly to target other combatants. Examples here might include a small-boat swarm attack targeting a guided-missile destroyer, or a hundred-dollar rocket propelled grenade fired at a multi-million dollar attack helicopter.

This term has be employed previously. Please note the following examples, my emphasis added.

Clinton Administration - White House Press Briefing (1997):

The President is outraged and saddened by this morning's incident in Hebron, when an off-duty Israeli soldier fired into a crowd of civilians. The President has called Chairman Arafat to express his condolences to him and to the families of the victims. The President condemns this cowardly act, which was clearly designed to make it more difficult to conclude an agreement on Israeli deployment from Hebron.

President Reagan - On the Bombing of the United States Embassy in Beirut (1983):

Let me begin with a brief statement. As you know, our Embassy in Beirut was the target this morning of a vicious, terrorist bombing. This cowardly act has claimed a number of killed and wounded. It appears that there are some American casualties, but we don't know yet the exact number or the extent of injury.

President Carter - On the Death of the Former Italian Prime Minister (1978):

My sympathies and the sympathies of all Americans go out to Aldo Moro's bereaved family and nation.

His murder is a contemptible and cowardly act. His death advances no cause but that of mindless anarchy. But his life was devoted to building his nation, and his political skills were forever at the service of justice.

In short, these acts are dishonorable because of the the mismatch of the combat status of the attacker and the target, rather than the methods employed.

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'.

But, this is not the only way of understanding the shaping of perception in the public sphere:

This is also media spin for constructing popular opinion. Rightly, I'd add, though I agree that in that regard it's not technically correct to use this word.

Taking a weapon and killing people is in one way not an act of cowering: the murderer (in these cases) will clearly be apprehended and their life will be over (through prison or death sentence). There is a strength (non-cowardice) required to more-or-less end your own life (literally or practically).

The public interest is served by not framing this as any-form-of brave. That would provide praise to the murderer(s). It would probably encouage others.

I have a personal hunch that the usage 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.