It has nothing to do with the airlines as it is a security requirement imposed on the airlines if they want to fly to the good old USA. If you do not comply the you will not be accepted into US mainland airspace.
Not just mainland it applies to Hawaii also. From Sydney, I've noticed that QF and JQ flights to the USA typically use the gates on the far left "arm" of the B concourse (gates 8-10), as these gates are more isolated, making it easier to set up the additional screening areas. Not sure about United or A380 ops as I'm not sure if gates 8-10 are fitted for the A380 or not.