In the midst of the "Ghost Dance" movement and just before and after the killings at Wounded Knee, Baum wrote in the Aberdeen Saturday Pioneer that his fellow white settlers' "only safety depends upon the total extirmination [sic] of the Indians." Prof. Waller Hastings's website offers full transcriptions of the remarks. [ADDENDUM: Link now broken.]
NPR's suggestion that Baum's statements "may have helped inspire" the killings at Wounded Knee is misguided. A small, failing weekly magazine of literary reviews and local gossip published in Aberdeen didn't influence US cavalrymen 350 miles away. Baum's remarks were simply symptomatic of a widespread racism in American society. And Wounded Knee is only one example of soldiers killing noncombatants whom they perceive as (a) a separate kind of people, and (b) supporting an insurgency, as mass killings at Lancaster, My Lai, Haditha, and other historical examples show.
The only reason we pay attention to Baum's statements is that his later books, particularly The Wonderful Wizard of Oz, became a beloved part of American culture. Most people who edited small-town weeklies--or even big-city dailies--over a century ago aren't read today at all. But thanks to the internet, far more people have read Baum's anti-Native remarks in the last decade than read them as originally printed.
I think it's important for Americans to know that Baum published those genocidal comments, and to consider what they reveal about the US campaign against Native American ways of life. I don't know of any scholar who has presented them well in the context of Baum's life and work. Katharine Rogers's biography of Baum relegates them to an endnote. The index to The Baum Bugle indicates that the journal has never published an article about them. But I think the web articles that reprint Baum's remarks but say little about how they fit into the rest of his writing and life don't serve readers, either.
Baum also wrote on his Lakota neighbors in mid-1890 in his "Our Landlady" columns, a popular feature of the Saturday Pioneer. These have been compiled by Nancy Tystad Koupal for the University of Nebraska Press. They show the same mix of humor, cynicism, and sympathy that Baum used to report on other locals, including himself. His first remarks on the "Ghost Dance" appeared in those columns, and suggested that, contrary to settlers' anxieties, the Lakota deserved to worship as they chose. While reflecting his society's racist images of Native Americans, Baum's columns argue for tolerance. So why did he change his tone so strongly at the end of 1890? Eric Gjovaag and I discussed the possibilities in an entry of his Wizard of Oz FAQ.
Hardly ever noted in discussions of Baum's editorials is that the "Ghost Dance" movement was itself based on hopes to (to echo Baum's words) "wipe these untamed and untamable creatures from the face of the earth"--those untamable creatures being white Americans. Genocidal sentiments are unfortunately quite widespread in history.