You know what, you are making these conclusions based on a news story.  I believe that the police would have arrested the dude if they had clear evidence that he had "lured" the boy away.  Recall he was found some distance from the mother.  It would seem that the mother did not just turn away for a moment.  How long was she busy talking?  Sounds to me like she left the developmentally disabled youth alone to his own devices for some time and he wandered away or got curious about something.  In any event, I believe police are only too eager to arrest and detain.  I trust that the reason the dude was not detained relates more to the absence of incriminating evidence - despite what the news story leads one to believe.