Because of course you know someone who said it happened to them. You didn't see it yourself. It wasn't you. But someone told you it happened. Because you weren't actually there.
You know, you should get in contact with those friends of yours so they can call a guy named Jerry Lembcke. Jerry has been trying to find anyone who was a veteran who was actually spit on. He's been doing it since the Vietnam war when the stories started popping up. And he's never found one. Not once. This whole time. This would be huge for him.
Here's the book he wrote on it back in 1998. https://www.amazon.com/Spitting-Image-Memory-Legacy-Vietnam/dp/0814751474
This would be a huge landmark thing for him. You should really make the effort to make it happen. Because then, finally, after all these decades it wouldn't be a myth but a true thing.