I think he wrote the book so that students didn't have to spend money on a huge textbook containing information they don't need. This book had everything you needed for the class and not much more.
Not trying to tell you that there aren't some jackasses who do this for extra loot, but one good reason to rotate the problems is to prevent students from showing up with an answer key. ie: When I was in college our IEEE club had a more comprehensive question bank than any single prof at my uni.
Another economics professor produced her own reader/workbook which was provided for the cost of photocopying and binding.
That kind of thing would be a free download today. Can you imagine paying thousands of dollars in tuition for a course, and then getting the entire textbook for free?
Those who do it are truly saints among scoundrels.
A small group of us got together that first evening and found enough typos, spelling and punctuation errors that we all passed the class with a high "A" without needing to do anything else.
He wasn't happy about it, and I think he later dropped the "unlimited" part. He was less upset when most of us went to class anyway -- The way I saw it, I paid for the class, even if I didn't have to worry about doing the work for a grade, I still wanted to learn something.