Indeed, but it does sound a little stingy when they explain it. The crewmember the OP mentionned provided the explanation about the taxes... He was rude doing so and that's a specific problem, but I feel there is no way to properly explain to a customer who potentially paid thousands of pounds for his ticket that serving diet coke before taking-off is a big no-no because they're trying to save VAT on the can. The astute customer will just take a can from the lounge, so there is very little problem wi that, it's just that the explanation is a little silly-sounding.