Because all three GSS aircraft where first operated by Atlas pilots for years from what we all know
Ownership of the aircraft is not relevant.
At the end of the day, it is UK cargo flown for British Airways. UK (or European) pilots have a more than reasonable expectation of being able to fly it.
Just as Qantas pilots have a reasonable expectation that Aussie pilots should be flying Aussie freight handled by Qantas.
Atlas Air and its American pilots make a significant living out of flying foreign freight. Reciprocal opportunities are extremely hard to find...
PG