The combination of the two LIMITS what the Federal governemnt can own - it has to be either part of D.C, or a "fort, magazine, arsenal, dock-yard," or other "needful building", and it has to be with the consent of the legislature of the State in which it is in. Otherwise, if the government can just own anything, why have the Section 17 in the first place? Since you have it, and the Constitution it was in was modified by the Tenth, the right to own parks, forrests, wilderness areas, etc, is reserved to "the States...or the people thereof..: That's why the Oklahoma land-rush and various other land disbursement schemes were enacted - the Fed COULDN'T retain ownership of the land, especially after the teritory became a state! Then Teddy Roosevelt came along, and decided not to tet the law of the land ruin his fun....