Matrika52, the AMERICAN Indians were certainly polygynous, even outside the plains. My lineage includes a Chickasaw chief, William Colbert, who may have been polygynous, and his father, James Logan Colbert, who was polygynous. Here is where I think it gets more interesting. James Logan was actually Scottish and married Native American women. Though the Chickasaw society was matriarchal with regards to leadership (chiefs were selected based upon mothers, not fathers), the marriages were still polygynous, not polyadrous.
With regards to polygynous Indians from India and the article above, like the others, I found it interesting. I find government marriage restrictions based upon religion to be a real oddity.