I'm guessing the Marshal didn't want to display his corruption to the "dirty water of the American imperialism"¹ even to the workers who stocked and served his beverages.
¹As the members of our marxist-leninist-maoist party called it ;)
Given the tolerant friendly loving environment of their government in that era, I'd prefer drinking clear drinks. There are plenty of 100% soluble clear poisons, but clear drinks would exclude at least SOME poisons, or at least make prospective poisoners work a little harder.