Morality of Using NX

You missed the point. By buying the cards, you've given them money. You could throw the cards into the fire for all they care at that point as they've made their money.

Well, thing is

The second the cards are activated Nexon has already made the money. Shuffling it around a little isn't giving them money, as they already have it.

If the game cards' demand is high, then 7eleven and Target will be buying more of them, as they're obviously something people want. Therefore, Nexon will make more cards, and be able to sell them. If 7eleven and Target doesn't sell the cards, they will not be buying more cards, and Nexon won't earn money by making new cards as noone will buy them.