There is a notion of "Praire Tough". It seems to have originally referred to the prairie soil, with a network of grass roots that broke conventional ploughs, and was then used to describe the tough and hardworking farmers who worked the prairies. This is an image that has been previously used in American literature:
... he was brought out here as a young bride... she seemed as if she was made for the prairie tough... nothing fazed her... designed for solitude and wide open spaces. (Home is where the Heart is, Linda Byler)
Traditionally it may have been associated with a certain kind of machismo: The ideal of praire tough would be a white, male farmer who, with gun and plough, subdues the land.
Now, logically there is nothing wrong with the statements:
North Dakotan women victims of abuse are North Dakotan (Tautology)
North Dakotan women victims of abuse are "Praire tough" (the claim)
It does not follow logically that "All North Dakotans are Praire tough".
Rhetorically this links a notion that makes North Dakotans proud to a particular subgroup, and implicitly suggests that abusers are not as "praire tough" and so are only North Dakotans in name, and not in attitude. It explicitly includes the Native Americans victims, to make the point that men that abuse Native American women should face justice within the tribal system.
This is rhetoric. The significance is that women may have been seen as being victims, and so weak and not tough. The letter writer is making the opposite claim, and using rhetorical techniques to do this.