Why do styles guides have what not to do?

I'm tasked with writing a style guide for a .NET application. Currently there's about six developers that just do whatever they want in terms of style. Similar buttons are in different spots, different controls are use, etc., etc. It's a mess.

I've researched some other style guides, and they often (always?) show what not to do. The lead developer mentioned that the style guide will be better received if I omit any "don't do's" and only include the "do's".

Why do style guides have what not to do listed as well?