The Infamous Eagles Joke

You should be warned that the following joke can be infuriating. It is said that it was used at Cambridge University as an intelligence test for undergraduates - those that got it were intelligent. If you get it - well done. If you don't get it don't worry - neither do most people when they first hear it. It is a regular (though rather subtle) joke which requires no special knowledge for its understanding. Here it is:

Two climbers were climbing a mountain. They saw two eagles soaring above them. A little later the climbers slipped over the edge of a precipice and unfortunately plunged to their deaths. Their souls left their mortal bodies and ascended to heaven. As they rose the souls saw the same eagles and one soul said aloud, 'Aah - Eagles.' But the eagles, being polite, said nothing.

I don't get it - give me a clue please