The Great Eagle Mystery

[SlinkerStinker]

I don't recall any instances of people actually riding on Eagle's backs - they always seem to have been carried by the arms.

In The Hobbit, when the eagles return the group from their eyrie, they let at least Bilbo ride on the back of the eagle.

This time he was allowed to climb on to an eagle's back and cling between his wings.

Also from the hobbit, the Eagle Lord in speaking with Gandalf says that they won't fly them near men less they think the eagles are coming to steal their sheep and shoot at them. If they are that adverse to flying into the "danger" of a lone sheppard, flying into Mordor would have been out of the question until Sauron was defeated.