I think this has to do with that recruits often use the religion as a substitute for whatever they were doing to mask problems. Usually, they are having problems--and the religion is supposed to take them away from those problems. And, the more they do, the more it has the illusion of doing this. You put yourself away from the world, and the more you do that, the more those problems are masked. Trouble, doing this is no better than using drugs to do the same thing.
With born-ins, they really didn't have much choice. They get dragged out in field circus when they are babies, dragged to boasting sessions, told what to say and do while at the Kingdumb Hell, and not allowed any other viewpoint. And the religion is counter to what most children naturally are drawn to--something adults do better at suppressing. A born-in might hear a "bad" song and like it because they are naturally drawn to it, while a recruit might associate it with something in the world (with help from the hounders) that gave them trouble and be drawn away from the same song. Same for things like field circus (more and better gets them away from their trouble), TV shows, reading material, and holidays (associated with the worldly problems).