That is the crux of the problem. Kids "earning" a higher GPA without ever actually learning the material by virtue of turning in complete and 100% correct homework. A good teacher would suspect a serious problem if such a student crashed and burned when taking an exam on the same subject matter in the classroom.
I taught college electronics and microprocessor programming/design classes for 3 1/2 years before my full time job made it impossible to teach in the evenings. My students ranged in age from 15 to 67. Skill levels, experience and aptitude were likewise wide ranging. There wasn't a "teacher's" edition for my classroom text. Fortunately, I didn't need one.
There are too many other ways for kids bent on cheating to cheat.
The bottom line is when they cheat, they really only cheat themselves. At some point they're going to have to prove themselves in the job market - and there they will pay dearly if they really don't know what they're doing.
The book company has every right to restrict sells to whomever they like. But once the book belongs to someone else, they too have the right to pass it on to whomever they like.