I was asking myself the same question, and I have reread the rules especially for this. The answer, for me, is that nothing in the rules forbids it, contrary to RQ3, where rune levels were explicitly forbidden to be shaman except where explicitly authorized (or even required). I think this is a good point that the rules don't prohibit it.
Of course. The rules should not forbid it, but the world has to give guidelines and rules, absolute or relative. It is logical that most cults restrict the activities of their members, especially the high ranking officials as rune priests and rune lords. This makes the cult book mandatory to know the details.
Same for me.