The fact that they realize, at some deep level, that their children MUST be indoctrinated points to a fear of questioning. They are not going to take the risk of letting a child make their own decision at the age of, oh, 18-ish . . . because by then it's too late. If your beliefs won't stand up to scrutiny, then they should be discarded.
"The family that prays together...is brainwashing their children."- Albert Einstein