They ceded their territories to the confederacy specifically named "The United States of America" ("The" does not operate as 'the' in that context--"The" is part of the name). The Northwest Ordinance and the related series of laws between 1785 and 1788 or so were rules for handling the territories. The non-imperial nature of The United states of America (i.e. the singular confederacy) and of the United states of America (i.e. the several states) seems to be apparent in that all territories were to become states rather than remain territories or possessions. Such may have changed with time and it seems mainly with the Civil War and the assassination of William McKinley (which some suggest to have been a coup). The French ceded the Sovereignty and Domain of Louisiana over to the United States with certain stipulations. In any case, the U.S. government like any trustee would be in breech of trust if it failed to adhere to the implicit or express terms of the trust agreement. In the case of large size of Federal land holdings out West, it might have something to do with the fact those States were formed after the Civil War. If the U.S. was operating only in its "Article I" or "Article II" capacity or capacities only after 1862 perhaps it could only offer limited services after that point like a food shop with certain equipment inoperable?
Relevance of the Royal Proclamation of 1763 to U.S.A. land history.
Relevance of Treaty of Cession of 1803 (i.e. Lousiana Purchase) to the U.S.A. land history.
Under the Treaty of 1818, this would be a joint British-American area for at least 10 years.
Consider land in Alaska as a case in point. Washington and Oregon were part of an area that was a "joint" between the UK or Great Britain and the United States. California (i.e. Alta California or Upper California or even Nueva California) itself was a territory of Mexico at some point before it became under the United States. Maybe the Government of the United States (i.e. the Executive) was able to rename Mt. McKinley because of this.The Treaty of 1818 set the boundary between the United States and British North America along the 49th parallel of north latitude from Minnesota to the "Stony Mountains"[2] (now known as the Rocky Mountains). West of those mountains was known to the Americans as the Oregon Country and to the British as the Columbia Department or Columbia District of the Hudson's Bay Company. (Also included in the region was the southern portion of another fur district, New Caledonia.) The treaty provided for joint control of that land for ten years. Both countries could claim land and both were guaranteed free navigation throughout. Source: Wikipedia
Keep in mind that the BLM was formed in 1946.
Related:
Our Hidden History of Corporations In the United States