Generally, if you receiveR$600 or more in gambling winnings, the payer is required to issue you a Form W-2G. If you have won more thanR$5,000, the payer may be required to withhold 28% of the proceeds for Federal income tax.
Because Modern Warfare is published by Activision, which is a part of the bigger Activision/Blizzard company. So to save money, they just put it out on the Blizzard launcher (Battle . net) instead of having to pay Steam to sell it here and they keep a part of the profits.