An excerpt from his post It's Because of the Book:
The reason why they are so easily victimized is because of the book, specifically in this case the Torah.
The problem arises because while less-fundamentalist versions of Judaism don't necessarily follow the Torah, they still venerate it. They still believe that it was God's divine gift. And they teach this to the kids. The book is important.
Now when the kids grow up, they start looking around and they find some people who actually take the book seriously. Perhaps they say, "If it's God's gift to humanity, then, shouldn't we be taking it seriously?" Because they don't necessarily have the tools (Hebrew) to examine it themselves, they're susceptible to cherry-picked verses and explanations. They can be presented with a very fundamentalist viewpoint, modernized by out-of-context quotes and sketchy interpretations (italics mine). And they eat it up. They eat it up, because they have been taught their whole life that the book is the key.The only solution, it seems to me, is to create awareness for what outreach does. Rebecca M. Ross does a great job at Jewish Outreach: What Your Rabbi Isn't Telling You.
Also, a positive outlook on skepticism, knowledge about the religion and the tools of critical thinking should help parents to inoculate their children against the dangers of (especially deceptive) kiruv.