Doesn't have to be all seeing to spot a fucking eagle lol. This is akin to "Gandalf should've teleported the ring to Mordor, it never explicitly said he couldn't"
I think that one's pretty well explained (albeit not explicitly) by the presence of the Nazgul and the eye of Sauron, which were either destroyed or otherwise occupied when the eagles made their rescue. People pretend Mordor had no airborne defenses for the bit, but it doesn't really make sense