@kensanata there're feedreaders (well, i know newsblur at least) where they make an effort to grab fulltext from the site when it's not available in the feed, but i agree it'd be nice if it were always there...
i'm not as sure, though, that it's a conscious choice in a lot of cases. i can think of a good number that don't post the fulltext who definitely aren't interested in ad revenue or analytics.