You're paying for the account when you buy the game.
Look at it this way. When you buy the box, it comes with a CD-key. You use that to make an account. The account is what you are paying for, and you get a month along with it. The account costs 50 bucks, the month costs nothing.
When you buy a car you get a tank of gas. The car costs however much, the gas is free.
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