For those who see no issue with wearing headphones here are two examples that have happened in recent events.
10 mile 3 lap course
Runner wearing headphones at he end of the third lap failed to hear or notice the marshal shouting at him to turn left into the finish area and proceeded quite merrily past the finish straight to start a 4th lap - no doubt humming a tune.
Half Marathon trail in winter
Several runners failed to hear several marshals warning of ice on a turn ahead and fell.
Also witnessed many incidents where ambulances have been attempting to get through runners to attend to an injury but runners haven't moved out of the way, not to mention general warnings that are often given by fellow runners during an event being missed - hole in road - car on left etc.
There are valid reasons why headphones should not be worn and if it is a condition of the event then it is selfish and irresponsible to think that they dont apply to you.
When I suggested that we spent 2015 Christmas and 2016 New Year in LA, I received one of 'those' looks from the lovely Lizzie W, a sort of Paddington hard stare. Not, I suspect, out of annoyance for failing to consider family commitments but more from the prospect that I wouldn't be providing yet another 90 minutes of entertainment on New Years Day by attempting to scale Boxhill and the Eiger Steps in some ridiculous form of attire along with several hundred other lunatics.