Because they will only produce what we as a society will accept. They will only spend money producing products that we'll buy. For this reason, as a parent, I never will allow video games in my home, computer or otherwise. You and I, in an indirect manner, are to blame for what happened in Colorado. We, as a society, must create and define standards of what we will and won't accept in our entertainment.
Free speech is critical, yet so is the right to live.
Both are required in a democracy. Choose your entertainments carefully, choose your time with your children carefully. Now that you've read my angst, get up off your butt, and go spend some time with your children. Go into their room, sit on their bed, and talk to them. Take them to lunch from school one day.