The expedition was itself a violation of the 1868 treaty.

The Black Hills were within the Great Sioux Reservation, and the treaty prohibited unauthorized persons from entering or settling there.

The army had, for a period, attempted to enforce this provision — turning back civilians who tried to enter the Hills.

The Custer expedition was different: it was the military, sent by the government, into territory that the government had promised to protect from exactly this kind of incursion.

Why the government authorized it is not mysterious.

The depression of 1873 had been severe.