The whole point is that pre-clearing does not take away from or add jobs to any US carrier, it makes life easier for the passengers. If you've ever had to clear in JFK you'd now what a difference it makes if you were able to get done with that hassle and pre-clear here.
If you read the APC thread you'll see one of the bones of contention is the US carriers cannot compete with us and other carriers service-wise, they pretty well suck in that department, their angry grandmothers and cat ranchers actually believe that they are there only for safety reasons and do not need to provide a service.
Their best ranked was Delta at 57 while Qatar was 1, Etisalat was 6 and Emirates was 8.
The World's Top Airlines | Ranking 1 to 20