One reason that I can see is that given time and a fixed examination question bank, all the questions will be known. The exam will merely become a matter of knowing the questions and not the theory behind the subject. Putting some compulsory study into the package at least ensures people have had to study the full subject to some degree. For every one person willing to study, there are hundreds who just will do the bare minimum. When I went through training a fellow student came back the night before the exams asking what he thought he should brush up on! Strangly enough he failed!
This is a method of regulating the theory and yet allowing for a fixed and multinational exam bank. Question papers are extremely hard to write, just look at the first few JAA exam series, before the wrinkles were ironed out!!