Valve: don't perpetuate stereotypes

This is specifically about the Valve games Left 4 Dead and Left 4 Dead 2.  These are first person, post-apocalyptic zombie shooter games.  As I'm playing through, did I mention I'm white?, I keep being distracted by the o-come-on things the black characters say.  The race ratio in this game is 5:3; the 3 black characters are the only ones who proclaim "I'm gonna grab everything I can!".  None of the whites and all of the blacks say this.  The whites'll say things like "Pills here" "Ooo a little pick me up", they actually have personalities.  
I'm asking Valve: care to explain this?