Virtual currency is a mean for the player to purchase content within the game. It's not an end by itself.
When I purchase an eShop giftcard, I'm buying virtual currency with real money, but in order to then acquire a digital product. The trick here is, even though they irrevocably take my money forever, and they even label the transaction as a "purchase", I'm not technically buying anything, since there's no transfer of ownership according to the TOS. I'm essentially paying for a movie ticket (in this case, a game ticket) that gives me access to the content indefinetely, but doesn't give me any sort of guarantee as a consumer.