Hmmn - he goes to teh bar, finds out is an “open bar” which implies that those bellying up to it are not charged for what they drink.
The writer never wondered if the bar was paid for by a particular group which had not invited him - and his lack of an invitation would seem to support this being the case.
Thus, when the first person came over and asked him to leave, there would be some reasonable chance that the request came from someone who was aware that Alterman was consuming drinks which were not supposed to be given to him.
But, “free drinks” is an irresistable lure to lawyers and writers.
Adendum to my last post - legitimate journalists won’t even accept a cup of coffee, lest they be seen as corrupt.