I should have stated that Dred removed the right of Congress to legislate on slavery in the territories. Everyone except insane abolitionists was at the time in agreement that Congress had no power to legislate about slavery in a state.
No. That does not work any better. The Court said: "The Territory being a part of the United States, the Government and the citizen both enter it under the authority of the Constitution, with their respective rights defined and marked out; and the Federal Government [60 U.S. 393, 450] can exercise no power over his person or property, beyond what that instrument confers, nor lawfully deny any right which it has reserved."
The Congress never had the power to legislate on slavery in a state or a territory. The determination of whether, or not, to have legal slavery was not delegated by the people to the Federal government. It was reserved to the States or the people. The Federal government had no more delegated authority to prohibit slavery in the territories than it had a right to abridge free speech or a free press or to establish a religion or an income tax.
The problem here is not with Taney but with the Constitution. It recognized and protected the institution of slavery. However wrong or immoral that was, the Court was required to acknowledge that and rule in accordance with the law as it was, not as they thought it should be. When the law is wrong, the law must be changed.
After enumerating various Constitutional rights or restraints upon the Federal government, the Court said: "The powers over person and property of which we speak are not only not granted to Congress, but are in express terms denied, and they are forbidden to exercise them. And this prohibition is not confined to the States, but the words are general, and extend to the whole territory over which the Constitution gives it power to legislate, including those portions of it remaining under Territorial Government, as well as that covered by States. It is a total absence of power everywhere within the dominion of the United States, and places the citizens of a Territory, so far as these rights are [60 U.S. 393, 451] concerned, on the same footing with citizens of the States, and guards them as firmly and plainly against any inroads which the General Government might attempt, under the plea of implied or incidental powers. And if Congress itself cannot do this -- if it is beyond the powers conferred on the Federal Government -- it will be admitted, we presume, that it could not authorize a Territorial Government to exercise them."
Everyone except insane abolitionists may have agreed that Congress had no power to legislate the legality of slavery in a state. This is because the people never delegated any such authority to the Federal government, and what was not delegated was retained by the States, or the people. Neither did the people delegate any power to the Federal government to legislate the legality of slavery in a territory. If it was not delegated, it was just as reserved as the power over States.