When we are calculating FCFE starting with Net income available to common, if Preferred dividends were already subtracted when arriving at Net income available to common, no further adjustment for Preferred dividends is required. However, issuing (redeeming) preferred stock increases (decreases) the cash flow available to common stockholders, so this term would be added in.
In many respects, the existence of preferred stock in the capital structure has many of the same effects as the existence of debt, except that preferred stock dividends paid are not tax deductible unlike interest payments on debt.