Here's the reason I think why bible thumpers make their kids go door-to-door proselytizing:

It's not so much as to gain converts to their religion, but rather to strengthen the resolve of the kids who are going door-to-door. I mean, if you can go through 3 years of people slamming doors in your face and telling you to fuck off, and you make it through, chances are you're gonna be a member of that church for life.

I think people that have done that see it as a badge of honor and by remaining memebrs of that church, they are surrounded by others who have had the same experience. this generates comraderie and is used to further strengthen the member's committment to their church.

In any event, hard-core religious types are fucking crazy. Just spend 5 minutes talking to one of them about God and why their religion is better than others. Wow.