I recall exactly the same about the movie, that Gaius Helen, the old Bene Gesserit who tested Paul Atreides (by putting his hand in that scary box) was tonsured bald. The passage in the book, according to the quote in the question, was different. Let's put that aside for a moment though.
In the book, and the movie, Lady Jessica and Princess Irulan were trained as Bene Gesserit. Neither were tonsured bald in the movie initially. That changed when Lady Jessica drank the liquid (I forgot the name, it was a sandworm exudate, or perhaps melange-related), as part of accepting the role of Reverend Mother for the rebel Arrakis community. At that time, several things happened:
- The transition was a traumatic process. That was clearly the case for Lady Jessica; in the movie, she screams in pain.
- Becoming a Reverend Mother caused Lady Jessica to give birth prematurely to her daughter, Alia. That was stated explicitly as a consequence of her transition in the book.
- Lady Jessica also wore one of those non-hairpiece things, around her now tonsured head.
This would be a rationale that would keep the movie consistent with the book:
Some women are Bene Gesserit, but not all Bene Gesserit are Reverend Mothers.
Once a Bene Gesserit becomes a Reverend Mother, perhaps her hair falls out. It would be consistent with the other traumatic effects endured as part of the transition e.g. spontaneously aborting. Similarly, one of the central themes of Dune was that only women were "strong enough" to drink the sandworm exudate, but that Paul would be the first male who could do this, yet not die. Also, the powers of Bene Gesserit include clairvoyance, maybe telepathy and psychokinesis. They were trusted advisers to royalty, led communities, like a religious order. Both are associated with physical asceticism, including tonsure and black garb, in our historical past. (Maybe the hair interfered with their mental prowess and faculties, so that it fell out, or at the very least, was tonsured?)
There are a few problems. The first is Gaius Helen of course. If she were a Reverend Mother, she would not have had hair in the beginning of the book. If she were not a Reverend Mother, she would have had hair in the movie. The situation was reversed though. I can't reconcile that inconsistency.
The other issue is Alia, Paul's younger sister. She wasn't a Reverend Mother yet she didn't have hair in the movie. Alia was a special case though. She was born prematurely, matured at an unnaturally rapid rate, had exceptionally strong Bene Gesserit powers, greater than even those of Gaius Helen. It would make sense that Alia wore the same black velvet gown and tonsure (though without that non-hairpiece) as the Reverend Mothers did.