Was that treaty ever ratified by Congress? Many treaties were not.
“Treaty with the Sioux — Brulé, Oglala, Miniconjou, Yanktonai, Hunkpapa, Blackfeet, Cuthead, Two Kettle, Sans Arcs, and Santee — and Arapaho, 1868” (Treaty of Fort Laramie, 1868). 15 Stat. 635, Apr. 29, 1868. Ratified Feb. 16, 1868; proclaimed Feb. 24, 1868. In Charles J. Kappler, compiler and editor, Indian Affairs: Laws and Treaties — Vol. II: Treaties. Washington, D.C.: Government Printing Office, 1904, pp. 998–1007.