“As they were designed to be in the 19th century, to keep the defeated Indians docile and “on the reservation.”
I’m sure there were many reasons, but the main one seems to be that they couldn’t assimilate with the WASP culture spreading across the west.
No, the Army was actually concerned about the spread of diseases like smallpox and tuberculosis, and wanted to keep the Indians separated to prevent epidemics as the general population moved West. War Department and BIA annual reports to Congress throughout the nineteenth century talk about this fear. Stopping the spread of communicable diseases from Indians is one of the reasons that the U.S. Indian Health Service was created in 1955, specifically to stamp out tuberculosis on the reservations (after it was found that too many Eskimos in the Alaska National Guard were infected and could not effectively guard the DEW Line).