The Eagles seem very picky about when they help and who they're helping. I highly doubt they would do such a deed without Gandalf letting them in on the full story.
Well, you can tell some Eagles that "there is a very important mission to go on with Gandalf, and we need some eagles who don't know why we're going on the mission."
Like, you could have the Army move a box of stuff for you, and tell the army, but still have none of the soldiers know what's in the box.
