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.
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.