I think it's the same reason some people call the shooting of John Lennon an 'assassination.' Sloppy language, and a desire to make it something more than it is.
#11
I see your point; in this case, I'd more suspect it's their desire to make it something LESS.
An "execution" is a formal thing with, potentially, something respectable about it.
This was nothing but a bunch of hatefilled cowards slaughtering an innocent Jewish/American civilian.
If it were reported that way... well heck, it might make people think we're at war with really despicable individuals! It might sound as if it were more important than a few idiots embarrassing a few enemy terrorists.
Can't have that.
< /sarcasm >
Dan