Morally there is a kind of symmetry between the two types of reparations cases: In general, there is no justice in blaming a group for the sufferings of another group when the first group does not by and large involve anyone with any direct tie to the wrongs, and where the second group is many generations removed from the sufferings. Groups as such do not have rights, and children are not to blame for the sins of their fathers. Only individuals have rights, and there are no obligations, in the Objectivist
view, that do not follow from the actions and choices of the individual. This is not to defend slavery, which was a horrific violation of human rights, or to exonerate the U.S. government in all its dealings with Native Americans.
There is a legal difference between the two cases, and one that has some merit.
In U.S. law, the difference between the Native American tribes and the descendants of slaves is that the tribes are and always have been seen as nations; what is being enforced in the courts are actual legal treaties with the tribes (as nations) that the U.S. government ratified years ago. The legal entities involved are the tribes and the U.S. Federal government. There simply is no such straightforward legal basis for the claims of descendants of slaves, as slavery was legal (if deplorable) until emancipation, and the U.S. government made no binding treaties with slaves in any tribal or national sense. That's why the law is different, in any case, regardless of whether there might be a kind of moral symmetry between the two cases.
Now, that is the U.S. case. I don't know much about the Canadian cases, although I have heard that some dubious evidentiary standards have been involved in some cases treating aboriginal treaty claims.
In essence, the treaty-based cases appeal to the terms of a contract that remains binding. The slavery reparations cases can make no such claim. Indeed, reparations for slavery would either require Congress to arbitrarily tax current citizens to subsidize descendants of slaves, or would require a court to find that someone with current standing had their rights violated during the period of slavery by someone who is currently alive. The former would be irrational and the latter appears impossible.
It is worth noting, however, that in a sense the U.S. Congress has long since passed slavery reparations. These take the form of the Great Society poverty programs, which disproportionately subsidized poor blacks, and especially in the form of affirmative action policies, which discriminate in favor of people from groups perceived to have suffered injustice in the past.
At TAS, we think that any legislature would better serve everyone by increasing the economic and political freedom of its citizens. Those citizens, whatever their race or background, best serve themselves by focusing on the future and developing the skills and character needed for success in a free society.