Go and sin no more

This is one I've never been able to figure out in all the years I've been reading the Foremost Newspaper of the Carolina: Why is it so all-fired hard to figure out what to do with an attributive proper noun modifier that ends in "s"? And now the illness appears to have spread across McClatchy to the Raleigh stablemate.

 Here is a really, really simple test. Substitute any proper noun that doesn't end in "s" for the noun in question:

The John Edwards saga
The Babe Ruth story

John Edwards' life story
Babe Ruth's life story

If you wouldn't make it "the Babe Ruth's story," why would you make it "the John Edwards' saga"?