Spitting was rare. However, disrespect did happen often enough.
However, most people knew that the soldiers were not the ones responsible. They had been drafted (except Trump, of course; he and his rich Republican buddies were kept out of harm's way).
The Vietnam war was the government's war; the American people did not want it.
Congress used it as an excuse to deprive Americans from things that made Americans proud (for example, the Apollo program). That war was VERY costly and just kept going.
As we sank deeper into it, it became clear (despite the media reports) that we were losing it, and that we kept using weapons that killed more and more civilians, usually while inflicting pain (agent orange and napalm bombs, for example). The US government used the armed forces to stop any protests (and even killed US university students just to make it clear).
When the war ended, some media (and the Republicans, of course) tried to make it sound as if we did not lose.
"We The People" knew better.