Schools are notorious for zero tolerance stuff. Just one of the many zero tolerance stories are of an elementary student getting expelled for the rest of the year for bringing a butter knife to school with his lunch.
It's a long standing tradition that parents, more often than not especially in todays world, are not rational people when it comes to their kids. Seeing as they are the one's threatening lawsuits and bringing all sorts of bad publicity to the district whenever something doesn't go their way, most districts would rather over react rather than under react and you get nonsense like this.