Because people are afraid of this shit, and unfortunately their fear is often legitimate.
If they kept having ball games, and some sort of racial violence happened (it's high school football, some sort of racial violence probably would happen), and the local news decided to pick up on it or a parent or an underemployed lawyer. Well, you see how this goes.
The same reason that some kid who brings fireworks to school gets expelled and federally charged with domestic terrorism. That shit happened around here a few years ago.
There is a very serious problem with what our public education system has become. Amazingly, I don't blame teachers for this one.