Doesn't seem like they're really doing this because of some big
conclusive discovery about diet soda.


In fact, I know the school close to where I live has several vending
machines, and these were a HUGE distraction to students. Lots of
leaving class to loiter by them, goofing around them, roughing the
machines up, etc.


Not to mention, diet soda already has a bit of a bad rep, being so
closely associated to soda sweetened in traditional manners. So, they
might just ban it ALL to try and make the push towards healthy foods
as powerful as possible to an observer.