It's all about SACRIFICE
The more a cult can make you sacrifice, the committed you will be to the cult. So asking for your time, or a promise to die for the cult or allow your kids to die (blood card), represents a huge SACRIFICE. People who have sacrificed for the cult are less likely to challenge the cult. For e.g. if you let your kid die over the blood issue, would you ever be able to admit it was all for nothing for a bullshit cult? Highly unlikely.
MORE SACRIFICE = MORE COMMITMENT