This is actually a feature in a WP Network that prevents an Editor of a site from affecting other sites. A user's profile is the same across all sites in a network, so if Bob is a member of Sites A and B, he just has to edit his profile once to change it in both places.
Say an Editor from Site A edits Bob's bio to say that he really loves crocodiles. Well Bob's profile on Site B would also suddenly say that he loves crocodiles. Because we don't want editors affecting other sites, WordPress doesn't allow profile editing.
But, of course there is a way to get around it if you are sure you need to. Check out this post/comment thread.