In fairness, it may not be as upfront as "not allowed" to leave. People may be told they can leave whenever they want, but that (if they do) they'll be a "quitter", a "backslider", they'll have "failed God/the movement/themselves". That's even before you get to organisations that practise shunning or disfellowshipping of people who leave.
So, it may not be as clear as "not allowed to leave" as the person being made to feel guilty or deficient for leaving, which is harder to measure.