The hotel should be embarrassed. Do they have a right to charge for the soda? Of course. Does it make sense to create ill-will over the price of a soda? Of course not. Obviously the OP is a good enough customer to have lounge access. As for monkey-see, monkey do - other customers seeing him take a soda so they take a soda - how would they know he didn't pay for it. The hotel staff could have gently stated something like, "Ordinarily soda is not included, but for you Mr. zzap, of course." That would have been a customer service win. Instead, we now all know the Westin in Taipei nickels and dimes guests.

FWIW, a couple months ago I asked for a Coke at breakfast in the Bali Sheraton. It was brought quickly, without question and without charge. It never occurred to me that there might be a charge. Since then I've given up soda and sugar, lost 30+ pounds and feel great, but that's another story.