Because the fictional cartoon character depicted on his T-shirt was armed with a fictional ray-gun.
Read and weep, for the mighty British Empire is about to expire killed by the liberal cancer from within:
Brad Jayakody, from Bayswater, central London, said he was "stumped" at the objection to his Transformers T-shirt.
Mr Jayakody said he had to change before boarding as security officers objected to the gun, held by the cartoon character.
Airport operator BAA said it was investigating the incident.
Mr Jayakody said the incident happened a few weeks ago, when he was challenged by an official during a pre-flight security check.
"He says, 'we won't be able to let you through because your T-shirt has got a gun on it'," Mr Jayakody said.
"I was like, 'What are you talking about?'.
"[The official's] supervisor comes over and goes 'sorry we can't let you through and you've a gun on your T-shirt'," he said.
Mr Jayakody said he had to strip and change his T-shirt there before he was allowed to board his flight.
"I was just looking for someone with a bit of common sense," he said.
(good luck to you, I say)
"It's a cartoon robot - what threat is it to security or offensive to anyone at all?"
A BAA spokesman said there was no record of the incident and no "formal complaint" had been made.
"If a T-shirt had a rude word or a bomb on it, for example, a passenger may be asked to remove it," he said.
"We are investigating what happened to see if it came under this category.
"If it's offensive, we don't want other passengers upset."