What I'm talking about are freshman and sophmore level courses, entry level, where the content hasn't varied significantly in quite a few years. My daughter teaches math at the college where I teach. I decided to take a trigonometry course, just to shake off some rust. The cost of the text, which was about 300 or so pages, was $125. This was in 2003, and the text was a 2003 edition. I asked the textbook publisher how often this text was revised. He said every two to three years. I found a previous version of the text, and the graphics and assignments were changed slightly, but other than that, the only real change was to alter the text so that the page numbers didn't match. It was strictly a change made to prevent resale of used books. Entry level trigonometry simply doesn't change enough to require a new version of the text every two years.
The "scam" is implemented on several levels, and has several components. The first is that the companies which produce the texts know that in a two to three year period, text sales will go down because of the supply of used texts on the market. Therefore, they revise books that do not need revising, simply to keep sales up. There's nothing wrong with this, except that the student is caught in a monopoly situation. Specific texts are required for specific courses. Many of these courses are required for specific majors. Therefore, the student MUST buy this book, no matter how ridiculously it is priced. Second, the college either runs the bookstore or leases it to an entity that runs it for them. The markup on the book after it reaches the bookstore is generally exhorbitant because, once again, the college has a monopoly on resales. Very large colleges, like the University of Texas, where I went, sometimes have competing private bookstores, but smaller colleges generally have the field to themselves.
I've noticed that college professors (no offense) are frequently totally indifferent to the cost burden on the student. Many instructors I talk to have no idea how much the textbook they assign costs the student and do not consider text pricing at all when selecting assigned books. I KNOW that there are texts for basic Trig that cost less than $125. But, like medicine, the people making the decisions simply do not consider the price when selecting the text. Students frequently complain to me about classes where the instructor requires several textbooks, sometimes each one costing $100 or more, and make no assignments out of some of the texts.
Some instructors assign their own texts to students, although I know several colleges that prohibit this practice. I've heard rumors of textbook manufacturers kicking money back to deans to assign specific texts, but I've never seen this actually happen. According to what I've heard, department heads, who decide on texts for multiple sections are generally the targets, because most colleges don't allow individual instructors to select a text, but require all sections to use the same one. I don't think I've ever heard of individual instructors getting kickbacks, but book publishers are a lot like pharmecutical sales reps, and I will admit holding up some manufacturers for Powerpoint presentations, instructor texts, and test banks.
Scam was probably not a good word to use. Inefficient, overpriced, and done with total indifference to the needs of the students would be more accurate. It's not one big thing, it's a bunch of little things. As a professor, you may be able to shed more light on it, but that's how I see it.