This is the one thing that should be made out and out illegal -- version churning without a fundamental reason.
Inclusive of: (1) altering problem sets or numbering, (2) reordering chapters, (3) organizing chapters.
Where if any of the above are done, a free mapping must be available on the internet.
I presume there are actual university guidelines prohibiting using older editions (i.e. ones the official college bookstore couldn't buy in bulk), but most professors just didn't seem to care (in between everything else they were juggling).
For my Newtonian Physics class, the textbook we were using was on its 14th edition and cost more than $100 new. I was fortunate that my professor asked us to use the 13th at the latest, and gave the mappings to the 12th edition problems (because the only difference between 12th and 13th was which order the problems were in). This for a subject that's been essentially the same for centuries.
Meanwhile my Computer Science classes all had $40 textbooks that were in at most the 2nd edition.