Well, the discussion is items, not services. But given that, the entire game is completely optional. But it's a game we're already paying for. The illegitimate part is asking people to pay again for something they've already bought and paid for. As a sub-based game, we're already paying for the entire thing. That includes all its items just the same as it includes all its quests and dungeons. Once we've bought the game and paid our subscription fees, everything the game offers should be available to get or unlock from within the game.
(Extra services like character transfer are a different issue, as that's not part of the game itself.)
Some people think it doesn't matter if we're cheated out of money so long as the amount of money is small enough to not have much impact on their personal budgets. Other people think the very act of cheating players out of money (by charging twice for the same thing in this case) is offensive regardless of whether the amount is significant or not. (It's sort of like people who don't think shoplifting minor cheap items counts as "real" theft.)
