Final answer:
The Sioux were upset about Mount Rushmore because it represented a desecration of their sacred Black Hills and symbolized the broken treaties and exploitation they endured as the U.S. government forcibly took their land following the discovery of gold.
Step-by-step explanation:
One significant reason the Sioux were upset about Mount Rushmore is due to the desecration of the Black Hills, which are sacred land to the Lakota Sioux people. The U.S. government broke treaties that had promised these lands to the Sioux. After gold was found in the Black Hills, the government, backed by military force, coerced the Sioux to give up this land. This resulted in a feeling of betrayal and exploitation among the Sioux. Moreover, Mount Rushmore's construction symbolized American dominance and the ongoing disregard for Native American culture and territorial rights. The work on the monument began without the consent of the Sioux and served as a constant reminder of the broken promises and continual erosion of their legal and spiritual claims to the land.