Technically, I guess you are right about "regular" troop units not being used outside the borders of South Vietnam. However, I was in an Assault Helicopter Company (119th) that was assigned the job of moving small and large teams in and out of Laos. Many other units were assigned the same task on a rotating basis under the Johnson administration. We were regular American units of the 1st Aviation Brigade. LBJ was a liar and those who swear to his lies are liars, and I would urinate on his grave.
As for the comment above, I knew "snake-eater" types back in the 60's and 70's who, if the comment were made to them that way, might more than disagree with you. My experience in late 67 was that extracting units while under fire from the "trail" area was the exception rather than the rule.
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The reason this happened was one of our top advisors was an Nort Vietnames agent, in fact a general in the North Vietnanese army. He gave away the operations and landing sites to the North ahead of time.