It's exactly this type of generalized authoritarian philosophy that keeps the War on Drugs going strong.
You can apply that argument to anything banned. Why ban anything at all, why not let kids drink in class? Because it causes a lot more harm than good, especially in a learning environment.
So, we restrict things. You might not agree where the line is drawn, but there is a line and its position is obviously subjective. I guess we can measure the effectiveness of this on learning and with feedback from the teachers though.