Yeah and one of them decided to venture into book writing, had connections and was friends with other professors...
That's one way to sell that pig lipsticks.
Edit: And incorrect about department = array(Professors)
That still doesn't make sense. If a department had a professor notable enough to write a textbook in a particular area, wouldn't they be teaching that course themselves? Rather than pressuring the other professor who is teaching it to use their book?
MATH courses typically have large student count, thus needing multiple professors and sections.