Politics had nothing to do with anyting here. Actually, the US 757 engine market is really split up with CO, AA (2end largest 75 operator), and US using the RB211. Others including National, North American, Eastern, which is not with us anymore, and ATA. Actually, the P+W2000 had developmental problems, thats why we see 757 operator like Condor, UPS, and ATA switching to RB211 powered 757s. P+W users include UA, DL (Largest 757 operator) and UPS (Now ordering RBs). There are actually few operators of P+W 757s in the US, you just see the 2000 popular in the US because these carriers happen to operate large numbers of them.
As for the RB on the 767, RR entered the game late, finally hanging RBs under 763 wings with, who else, BA. The only other original RB 763 operator is China Yunnan Airlines, which operates 3 examples. Currently, because of BAs efforts to lessen its fleet, over the fact of declining passanger numbers, BA is leasing RB211 powered 763s to QF (Quite strange seeing QF uses GEs on its 763s). Now, the main reason why the Rb211 is so unpopular with 767 operators, is the fact that the engine is substancially heavyer than its competitors, and because of this, cracks were found on the pylons on BA 763s. Modifications have been made to fix this.
Hope it helps.