[zeteo]

Except the evidence in the original story is clearly insufficient, and absence of evidence is not evidence of absence. We know nothing of Sauron's capabilities to detect and confront airborne intrusion into Mordor or about Gandalf's assessment of these capabilities. Assuming Sauron could not see or bring down the eagles and that Gandalf was aware of this fact is, from this point of view, adding new elements to the story and therefore fan fiction.