Earlier quoted context omitted.
I think that if the instructors are allowed to be lazy enough to use a test bank, then students can't be blamed for using it as well.
This is completely wrong. If the students are allowed to use the test bank questions, then it's ethical. If it's not provided to them, or they obtain it surreptitiously, then it's cheating. Practice tests are allowed for the SATs, but previewing the questions was clearly not allowed in this case. These rules are arbitrary , but real. Just because certain exams are open-book, doesn't mean all students are entitled to…
However, those sorts of courses are not typical, and it falls to the professor to make clear and explicit if external material is forbidden, and to explain why.
Since the existence of a test bank implies the exam was (mostly) multiple choice, the above concerns do not apply. There is no reason to forbid students from learning from external resources if your idea of testing them on their knowledge is giving them a multiple choice test.
The proper (and common) scope for restricting external materials is an individual problem set, quiz, or test. It's implied unless stated otherwise that you shouldn't get help for specific problems or questions from external resources, but it is almost never implied or stated that external resources are banned in the scope of an entire class. In that context, if you go exploring for materials and find something that helps you on a later problem set, quiz, or test, that's your reward for seeking out more materials to learn from.