“Nowhere is there authorization for federal public lands for any other purpose.”
Other purposes were contemplated, and provided for. Article 4 Section 3 Clause 2
http://press-pubs.uchicago.edu/founders/tocs/a4_3_2.html
http://press-pubs.uchicago.edu/founders/documents/a4_3_2s8.html
And the acquisition of territories by the federal government was never meant to be a permanent one.
§ 1320. As if it were not possible to confer a single power upon the national government, which ought not to be a source of jealousy, the present has not been without objection. It has been suggested, that the sale and disposal of the Western territory may become a source of such immense revenue to the national government, as to make it independent of, and formidable to, the people. To amass immense riches (it has been said) to defray the expenses of ambition, when occasion may prompt, without seeming to oppress the people, has uniformly been the policy of tyrants. Should such a policy creep into our government, and the sales of the public lands, instead of being appropriated to the discharge of the public debt, be converted to a treasure in a bank, those, who, at any time, can command it, may be tempted to apply it to the most nefarious purposes. The
George Tucker
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Which begs the question- If land acquired as a territory was to be sold for the purpose of reducing the public debt, why do we now have both a HUGE PUBLIC debt AND massive public lands?