Probably because the boy was under 18.
Age does not seem to be the problem here (age of consent is 16 in OK). Money exchange and being within certain distance from the church is the problem. These preachy types really have higher ratio of sex weirdos than general population it seems.
Age of consent merely means you cannot be charged for rape. The boy is still a minor prostituting himself, so you probably can get charged for all sorts of things.
I do understand the "trafficking" probably doesn't sound right, when you hear trafficking you think something out of that movie Eastern Promises. But likely, they way the law is written if it involves a minor it is automatically trafficking. Also, I read he recorded it, so they probably were going to get him on production and transportation of child pornography, and that probably entails child-sex trafficking.