They don't. I'd guess 95% of them couldn't find Serbia on a map. They are like lemmings going over a cliff; one goes and the others will follow.
You have to find a reason "why"? The KBR/Blackwater was a good angle and may be just that. Their hatred of most things American is tied to KBR. It has become their hate diety over the past 5 years.
Another angle may be ... Russia. Look for a leftist tie between Putin and the Hollywood elite. Shouldn't take long.
Maybe they just know that Serbians are good people. Most of them have been surrounded by them their whole lives. Serbs saved 500 airmen in WWII. Karl Malden is Serbian. Robert DeNiro lived in Serbia while he was young and danced at a Serbian wedding while filming Deer Hunter here in Ohio. Sharon Stone grew up with Serb Americans.
Americans of Serb descent have lived law abiding lives, assimilated and contributed greatly to this country, and are good people. They are NOT the savages or genocidal maniacs that they were purported to be by the likes of albanian drug money funded PR firms and politicians the 90s and this decade. Like the article states, there are many in the entertainment industry
Kosovo is Serbia! Thank you George Clooney and Sharon Stone and others!