Only shields obtained in Nightfall, EotN, and end-chests may be modded with a Shield Handle and possibly an inscription. Drops in the Fissure of Woe and the Underworld, as well as items from standard chests, will always be non-inscribed.
It doesn't say "of fortitude" because it does not have an "of fortitude" mod on it. It is called an "inherant" mod, meaning it was already there and always will be