[runningdogx]

If it were a process-oriented class, that might be justified. For instance, in a pure math course, you wouldn't want to read an off-syllabus math text in which the author proves theorems that show up on later problem sets or exams.

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.