Thanks for the interesting replies!
From what I've seen of the US airline security measures, the staff are usually supplied by local security firms under contract as and when required.
Ab400, you made the point also about "knowing" who is on the plane, so if there are any problems, they can be dealt with before arrival in the US. This couldn't be a reason for the security measures for one reason, foreign carriers don't have to do it. To make US carriers take extra pectautions to check who is on their planes would be unfair, as they would be doing the work of the US immigration authorities, and the US passengers would pay for it twice (ticket and taxes!) The airlines would complain like hell if they thought they were doing this!
As for US airlines being more secure, I'd say this was only the case when airport security is lacking. At Manchester, as I said above, there is 100% X-ray screening of all hold baggage, as well as hand baggage - one of only 5 airports in the world where this happens. It just seems bizarre that the US airlines have to spend all that extra money, when they are already paying the airport to provide some of the best security in the world in their passenger charges!
It may only cost 15c, but that's still $15000 per Million passengers on Atlantic flights. The cost may be insignificant on a per-passenger basis, but to the airlines it must hit their bottom line.
Just a strange situation I guess, you never think of the USA having to take security measures usually associated with Middle Eastern airlines
Thanks for your replies everyone