In legal terms the decision seems to be generally founded on logic and the law, with two exceptions. The first isn't actually crucial:
QUOTE
'In the opinion of the court, the legislation and histories of the times, and the language used in the declaration of independence, show, that neither the class of persons who had been imported as slaves, nor their descendants, whether they had become free or not, were then acknowledged as a part of the people, nor intended to be included in the general words used in that memorable instrument...for if the language, as understood in that day, would embrace them, the conduct of the distinguished men who framed the declaration of independence would have been utterly and flagrantly inconsistent with the principles they asserted.' (my emphasis)
This seems to be nonsense. It's based on the idea that black people couldn't possibly have been meant when they talked of high principles of liberty etc, simply because they were black. However, this is not the basis of the decision, but a way of trying to show that the decision is in line with the intentions of the law-makers. This isn't necessary, as it is the law that ultimately counts rather than the intention behind it, but judges like to do this kind of thing to give more weight to the decision.
More important is the dubious assertion that Congress, although entitled "to dispose of and make all needful rules and regulations respecting the territory or other property belonging to the United States," was acting outside its powers in creating the Act prohibiting slavery in the north. If slaves are property then I don't see how this can be. There is nothing in the constitution that confines it to the original 13 states. In fact that would be absurd, yet that is what they tried to say. Furthermore, it is also unclear why this process does not come within 'due process of law' as stated in the 5th Amendment, and therefore a valid way of depriving someone of their property. They are right to say that slaves have no different status under the federal consitution, but they don't need one.
My reasoning would have been this:
1. Congress is empowered to make laws concerning property;
2. The 5th Amendment allows for people to be deprived of property by due process of law;
3. The Act prohibiting slavery in the north is a correct law;
4. Therefore Dred Scott was made free by due process of law, under a valid law.
The Court's reasoning seems to have been this:
1. People cannot be deprived of property except by due process of law;
2. Movement from one territory to another cannot be 'due process of law.' (It must be done through courts?);
3. Therefore a slave cannot be freed simply by entering a territory;
4. Therefore the Act prohibiting slavery in the north is unconstitutional;
5. Therefore Dred Scott was not freed.
The problem with this is point 2 - it ignores the fact that the Act prohibiting slavery in the north itself made the freeing of slaves in free states a valid method of attaining freedom. That was a process of law.
So I actually suspect that Bush was right to use this as an example, though I admit it isn't totally black and white.