From the T&C if you scroll down on the provided link (under number 2):

Important Tax Information: Enrollment in Citi ThankYou Rewards and this offer limited to customers who are either citizens or resident aliens of the United States (U.S.) with a valid U.S. taxpayer identification number. The value of the rewards from redeemed Points may be reported to the IRS as miscellaneous income on Form 1099-MISC in the year redeemed, if the value of the rewards plus other taxable miscellaneous income awards received from Citibank, N.A., is in the aggregate $600 or greater for a calendar year, as required by applicable law. Customer is responsible for any applicable taxes.