Vietnam provides an easy vehicle to push you antiwar thoughts. It was an unpopular war, and it's veterans were mistreated by huge numbers of people. Some filmmakers are probably trying to stress the "horrors of war", but that often entails making soldiers in Vietnam look like savages, regardless of the truth.
Additionally, Vietnam marked the turning point in American opinion about war. Many people consider WWII to be the last "good" war. In reality however, it seems that the 1960's was simply the decade in which the United States embraced the concept of the counter-culture. In my opinion, this marked a sharp decline in the quality of America's youth, and ultimatly American society. Veterans of more recent wars, like me, have encountered resistance from protesters and non-veterans who seem to think that war is never a solution, and that it is "cool" to protest it, even if you have no fucking idea what is really going on.