They need to keep it.
It makes them 'special'.
There is another good reason too. Charlie Manson used it. Getting someone to do something that they would abhore as their normal non-cult self and the incentive to leave you, or betray you and admit to their peers that they committed, condoned and encouraged such acts without good reason because they were influenced by a nutter cult leader, stupidity, failure to do proper research on the creeps in charge, etc., is an extremely powerful control tool.