Combination of guilt because they work, and the pop psychology crap that says you should only say good things about your kids or you'll crush their spirit.
Pop psychology is annoying, since most of it is usually founded on theories that haven't been properly tested, if even tested at all. If individuals would actually take a look at the real, supported psychology, then they wouldn't have this problem with sheltering/coddling their children. They would know that it is an ineffective and harmful way of raising a child. There needs to be a balance between a permissive parenting style and a dominating parenting style and this really isn't anything new. Studies about what kind of parenting style produces the best results have been around for quite some time. But, like usual, individuals tend to ignore anything that is the least bit logical.
And it has already been proven that abstinence-only programs, and those silly
purity rings, not only don't work but that they come with an obviously much higher rate of teenagers coming away with more STDs and HIV infections since they were never taught proper means of protection and safety.
About intelligent design, well I'm going to have to agree with mando on that one.