The Northwest Ordinance was adopted in 1787 under the Articles of Confederation, which contained no mention of slavery.
The Constitution only gives the Congressional authority to ban the importation of slaves after the year 1808.
Thus, there was no legitimate authority to ban slavery in either the States OR the territories, only their importation into the country.
With respect to what has taken place in the N. W. Territory, it may be observed, that the ordinance giving its distinctive character on the Subject of Slaveholding proceeded from the old Congress, acting, with the best intentions, but under a charter which contains no shadow of the authority exercised. And it remains to be decided how far the States formed within that Territory & admitted into the Union, are on a different footing from its other members, as to their legislative sovereignty.
James Madison to Robert Walsh 1819
Then again, what did Madison know, huh?
So your belief was that congress could only be as strict in its laws for the territories as the most lenient state?
The terms of the Northwest Ordinance were reaffirmed unanimously by the 1st Congress, which included 16 of the 39 signers of the Constitution, and was signed into law by George Washington. But what did they know, huh?
Are you really going to defend Dred Scott as correctly decided?
Aside from the fact that the Constitution required the new government to respect all laws enacted under the Articles, the Northwest Ordinance was considered so important that it was adopted in its entirety by the 1st Congress, with only minor word changes to conform to the terminology used in the Constitution. It was among the first laws enacted under the US Constitution, and it passed unanimously!