Dirty Harry,
It is called a D visa and it is automatically issued to airline crews that are operating in and out but not in and in for America
It is the same visa that every single JMC pilot used. But it is not a visa that requires any sort of application or whatnot. Just showing up is all that it takes.
However, if you operate 1 leg inside the US, the picks up AND drops off passangers then you need the green card. You could pick up pax in LAX, go to JFK and pick up more pax (as long as no revenue pax get off, change of crew is okay) and then continue on to Europe. No Green card/work permit required.
A D visa is NOT the same as a green card or work permit. But neither is required in the above situation.
If they are restricting it to Green card and work permits only and they are only flying international service, then that is simply the choice of the company involved and not a matter of regulation. It would seam that they are doing what you would want to demand that the British carriers do. Maybe they just got more character, or a better supply of warm bodies.
Cheers
Wino