Personal credit card rewards are not income because they are a refund of your own money. An exception could be if the expenses which earn the reward are deducted from your taxable income (such as when making a charitable contribution, or an expense for business). In those instances the deductions may need to be reduced by the value of the reward, or if the deduction was taken in a previous year the partial reimbursement (reward) might be treated as income when received.
The card issuer would have no knowledge of such specifics, and you should discuss them with your tax advisor rather than rely on advice from an online message board.