It could kind of go either way in my opinion. Who is to say that said grenade wasn't damaged in the kill shot. Or isn't not so obviously available on the smoking corpse that someone getting shot at would have time to grab it? or so covered in gore that reasonable people aren't going to want to pick it up and throw it.
I take your point in that a lot of tactical RPG style games allow characters to loot corpses of fallen comrades in the middle of combat, but really? How probable is that.
From the developers stance, they probably figured that this was just another aspect of limited resources. Tactically, deal with it.