Call something a stylebook, and you conjure images of fashion photography and models. Add the words Associated Press to it and that gives it an aura of authority, like holy writ.
The AP Stylebook is a journalism referee, explaining what things mean and is embraced across the industry because it wasn’t always insane. It’s supposed to be the news media version of wordsmith Susie Dent, only without all the wit, humor or accuracy. (Dent might call the stylebook mumpsimus, a 16th century term for "someone who insists that they are right, despite clear evidence that they are not.")