There are two ways you can think of this.
1. impairement is on paper. You don't actually pay cash because of it.
2. even though net income is reduced as you said, but to arrive at CFO, (using the indirect method) you need to adjust the change in assets. Like inventory, an increase is a use of cash (deducted from CFO) while a decrease is a source of cash (added to CFO). Since asset is decreased by the amount of impairement, it is adjusted back.