Alcohol doesn't cause problems for 99% of people though does it.

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.