Actually, in economics, I find that marks often have a bimodal distribution, so that would not have aroused my suspicions. I have never worried too much about using textbook questions for proctored exams as there are usually so many questions that students would basically know the course if they learnt them all. Moreover, to encourage students to do the end of chapter problems, I usually tell them that there will be one or two from the end of chapter problems. However, it does create a problem for assignments as you can no longer just assign end of chapter problems.
I think that problems with testbanks and textbook solution manuals became much worse when they stopped only being hardcopy. Now that they are provided in soft copy form, it is too easy for them to get posted online. It only takes one instructor to give a copy to his TA (it makes it easier for TAs to help students during office hours with end of chapter problems) and that TA to give it to a friend, etc.).
Another problem is that in some disciplines, testbanks and textbook solution manuals have fewer questions. A friend in business tells me that the publisher in some of his courses only provide around 40 questions per chapter which is really not enough.