1. You have a game that "laments war" yet features killing as a pretty big gameplay mechanic
2. I can't think of any game in the series admonishes a player for killing. You're rewarded for not killing because in many instances it's more difficult to playthrough without killing. Particularly in GZ where you're given the option to kill US marines just doing their job, a first for the series
3. Maybe not so much in 4, but including RL manufacturers helps with immersion, especially with politics. Certain munitions indicate the nationality of your enemy, whilst Anericans with Russian munitions are meant to show some allegiance that's meant to confuse the story and set up plot twists
4. There's nothing inherently evil about gun manufacturers, so I hardly think think that paying a license fee to add a degree of realism undermines much of anything