Since the Kennedy assassination it has been US governmental policy to treat threats made against the US president very seriously. Threatening the president is treated about as lightly as saying you have a bomb in your backpack at an airport baggage check.

Still, I suspect someone in the chain of people involved in this story engaged in a bit of unauthorized embellishment. Perhaps an FBI official conversationally told someone at Scotland Yard (and that's the chain that would likely have been followed - the FBI doesn't directly talk to village police in foreign countries) that the kid would probably be banned. Then someone at Scotland Yard relayed the info to the local cops as the kid had been banned. Who knows?