a) why the hell does a person who's only doing word processing and emailing need a G4?

The one-time purchase price is a small part of what it really costs to operate a desktop computer. Even if it is a bit of overkill, businesses sometimes conclude that it is less expensive to support 100 identical higher-end machines that it is to support a mix of different (say 10 high, 30 mid and 60 low-end) machines. It makes them more interchangeable, for example.

b) why not go to OSX when you're installing all brand new machines anyway?

Good question. Is it possible that they are running a key app or apps that aren't supported on OSX yet?