Good for Serbia for implementing some centralized system. A PCR test result in the U.S has absolutely no way of being verified, either negative or positive In Israel, which swabs every person who lands at the airport. There is a suspiciously steady stream of people flying in from the U.S who were negative before take off and positive 10 minutes after landing
Google suggests that it takes 10-12 hr to fly from NYC to Israel. That's a long time with a fast virus. (Even with a week's incubation period, you'd still expect to see it, just less.). Furthermore, many of those negative tests will have been 24-48 hr prior to takeoff, depending upon legal requirements that day.
It's okay to suspect malice, but it may not be necessary here :).