One the one hand, yes, the kid's obviously a moron, but does anyone think Homeland Security has made America safer by denying him entry? I mean, I've seen his picture and read the tweet, and I'm pretty sure he's not an actual threat.
I'd guess that a trained immigration officer might come to the same conclusion, and if there was any doubt, they'd be able to call upon the investigative power of both the US and UK governments to make sure.
I don't think this kind of security theater actually makes anyone more secure.