The reason for it is because it's an Xbox 360 game.
Everything on Xbox Live! for the Xbox 360 was required to go through Microsofts payment portal, the transaction is then tied to your account, forever.
If it was a free Xbox One game, you wouldn't need to input any payment details, it is also why you can continue to *keep* your Xbox 360 games when your Gold expires, but not your Xbox One games through Games with Gold.

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