Amortization of Goodwill is not a tax-deductible expense. As such, it is a permanent difference in taxes. So it is not added back in that specific calculation since it does NOT reduce Taxable Income, resulting in higher income taxes since it is not deductible.
To answer your direct question, Amortization of Intangibles is NOT added back in that specific calculation because it DOES reduce Taxable Income and thus, one pays lower taxes. So really, the adjustment is for Goodwill (usually zero amortization anyway), not for other intangibles.