wiggy,
I believe that US 'Homeland Security' personnel have been working at UK/EU airports for a number of years. They are unarmed (thank goodness) and they have no official capacity other than to advise the airline/pilot that the aircraft would not be allowed into US airspace if PAX 'so-and-so' is on board. This applies both to aircraft destined to the US and those intending to overfly US airspace (i.e. Caribbean / Central America flights).
The airline obviously cooperates with the officials - what choice do they have?
Oh, this may be a secret. So don't tell anybody. Thank you.
OTOH would it have better for the passenger to have been refused entry at the arrival airport? (Or the aircraft required to fly around US airspace)?