You described it as "tax is removed from the value of the card". That was not true for me when I bought this gift card on Amazon - the $10 gift card is still worth $10 to Microsoft, but I had to pay sales tax on it (WA state). When Amazon sold the Xbox "points card", I didn't have to pay sales tax on it. I think this is just Amazon's error of mistakenly collecting sales tax on this new Xbox gift card, because the purchase of gift cards is actually exempt from sales tax. And even Amazon's sales tax policy says this: "Note: No sales tax is charged when purchasing gift cards; however, purchases paid for with gift cards may be subject to tax."
Either way, this isn't Microsoft being sneaky about getting more money from you. It's sales tax. Microsoft doesn't get to keep sales tax money.