Simple. He was in high school and still living with his parents when he first started developing and pursueing a interest in Islam,and he went to local Mosques to learn. It was easy for them to check his bonifides,and verify he wasn't a intelligence agent. There are very few teen professional intelligence agents still living at home with their parents.
Let's also not forget that it takes literally YEARS to reach the point of trust Walker had reached,and he started long before Bubba Bush took office.
Our government has been known to do such sneaky, underhanded things in the past. Why should we think this time is any different?