I believe that anybody has the right to use whatever platform they most desire to exercise their freedom of speech and their right to protest.
I admire those who seek to use video games for things the developers never imagined, such as platforms for expression and protest.
It is true that video games (all media, really) sanitize and romanticize warfare.
With that being said, if I get into a game of Unreal Tournament 3 or Halo 3 and somebody is engaging in a peace rally, one of two things will happen: if you are my opponent, you will be marked as an easy kill and you will cost your team a good deal of points; if you are my teammate, you will get a polite yet stern earful about how this is a game, and there is an objective to be met, and if you do not want to work towards that objective then you need to leave.
Sandbox games as protest? Brilliant. Criticizing a game for making war look pretty when war, is indeed, Hell? Perfect. Injecting protests in such a manner that it directly (and negatively) impacts somebody's game play experience? That's disrespectful.
Of course, there are ways to criticize said protests and there are ways to just look like a fool. Telling somebody to "go play with their Barbie" is the latter.