They teach on an 83 because the 89 is wayy too damn powerful. It does everything. But as a result it is also very complicated - it took me some six weeks to get completely used to it from my 85, but in general, the 89 is much easier to use.
Just like computer classes teach Word and Windows instead of Linux and CorelOffice, they teach what is simple. The focus of the class is teaching calculus theory and not how to use a calculator - which, if they taught on the 89, more students would have problems. But also, like I said, it's just too powerful.