Because:
1.) Fuck the troops. US soliders in Vietnam were every bit as genocidal as IOF troops today.
2.) The whole idea that Vietnam was holding any POWs after the Paris peace accords was a total lie made from whole cloth. There has never been any evidence presented that there were any POWs being held after the war and anyone who spends 10 minutes on the internet reading about it will come to that conclusion.
Seeing this flag burn, in emoji form, would be nice.
I never understood the whole "Vietnam still has our boys" narrative. Like, why? If they did bad things wouldn't Vietnam want to publicly try them for war crimes? Wouldn't they also want to be free of having to house and feed them? What possible intelligence could be gleaned by interrogating a guy like ten years after the war? Also Vietnam was busy rebuilding their country, neighboring countries, invading Cambodia, and getting invaded by China.
They were probably too fucking busy to bother with this whole nonsense.
Afaik it seems like something Nixon pushed before the war ended. But honestly, it seems like it carried on, at least through 80s, just because the concept was useful for making financially successful movies.