Xbox One Games Tax Free?

[coip]

Well, I finally bought a game (Killer Instinct) and to my surprise also found that I was not charged tax:

This mail is a confirmation that you have purchased the following items from Microsoft:
Purchase Date
Thursday, December 12, 2013
Total before Tax: $19.99
Tax: $0.00
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Total for this order: $19.99
Item(s)
Killer Instinct: Combo Breaker Add-On
Price: $19.99
Quantity: 1