There are as many reasons as variations on this theme. My stepdad made it a pretty-much-ironclad rule to never drink any beverage directly from its container, but to pour it into an appropriate glass, cup, etc. first. As far as I can tell, this was grounded in his adolescence, spent as a homeless orphan in Europe during WW II, when there was hardly ever a glass, cup, etc. available to drink from, requiring beverages to be consumed from their original containers.

I'm glad you liked the piece.

Cheers...