Well, I'm glad it's skippable, I think I'll probably skip it, I don't really want to put myself in the place of someone gunning down innocent people, especially not when it is presented in such a realistic way.
I wonder how much controversy it will cause if it gets in the news though? It could easily cause more furore than the GTA games, with people saying it is a murder simulator and terrorism simulator and is encouraging people to go postal. There's bound to be someone frothing at the mouth over it. Pretty shocked a company like Activision is allowing something like this to be published.
As for whether they should have this in the game or not, I really don't think it's necessary. They could just use cutscenes or movies or photographs or any other conventional means to convey the atrocity of what the terrorists do. Seriously, I don't need that much motivation to kill bad guys in video games, and I'm already extremely anti-terrorist, so what is the point of this level except for sensationalism? I really don't think it is justifiable in terms of serving the story or gameplay. We all hate terrorists already.