On the other hand, I doubt very much that the FBI found this kid posting comments on something harmless like a sports website and then lured him onto a terrorist supporting website. Odds are, before anyone took notice of him, he was already taking an interest in jihadi stuff on his own initiative. Not that mere curiosity implies guilt, it only explains why he was singled out for a closer look.
There aren’t enough investigators to monitor all the harmless websites out there (unless they relate to Trump) so they are going to be assigned to just monitoring the dark stuff, not Sesame Street.