IMHO your doubts are unfounded. The Servicemembers involved were under strict orders not to retaliate - especially while in uniform. The Pentagon was so cowed by the anti-war protestors and their allies in the MSM that no fight - no matter how justified - would have been portrayed in a fair manner.
As recounted in the letter above the documentation of "spitting" incidents is legion. Including the testimony of members of this forum.
It is like taunting a tiger in the zoo. The bars that prevented retaliation were the discipline that the soldier/sailor/airmen/Marine had inculcated in their training. By not fighting back against the smelly hippy he kept faith with his brothers-in-arms still in harms way.
Regards,
TS
Well said.