He didn't get
in - they fingerprinted him and cross referenced the prints on their database and saw that he was not the person he was claiming to be.
That is why he was not allowed in, and why America doesnt want to give him another chance. He was lucky he didn't get locked up in America on terrorism charges - he actually got a flight back somehow.
"He used a self check-in kiosk to board the flight at Heathrow and was allowed through when the document was checked in the bag-drop area.
But when Lennon arrived at New York's JFK Airport, customs officials took his fingerprints and realised he was not travelling on his own passport.
Lennon was asked to attend a second interview but managed to leave the airport, entering the US illegally."
https://www.bbc.co.uk/news/uk-england-20935502
If a "mad Muslim fundamentalist" tried the same thing they would probably be stopped on the UK side for further checks.
Have you never used self-check in for travel?
He is a criminal in so many ways - illegal travel, fraud, hate crimes, violence - it is shocking some people support him. But then, Hitler and Bin Laden had a lot of support, so I guess no real shock.