Do professors of math really write textbooks on basic math?
That said, you are right that my point goes primarily to college and graduate textbooks, where changing a few lines here and there qualifies for a "new edition" that makes all the other almost identical previous editions "obsolete." And I have no problem with professors or anyone else making money off their knowledge.
But I also have no problem with calling B.S. when there is a much cheaper path to the same knowledge.
Yes. Or they co-author or consult on the writing of a new math text.
It is quite the racket.