That's a very interesting thought, but also very odd in other ways. Most of that territory you are discussing was obtained by treaty. Thus, legally, one could argue that the territory thus was granted or given by treaty and legal treatise.
However, the ethics of it could also be called into question, that when a treaty is signed under duress or under lack of ability and knowledge, is the treaty legal?
The US would say that it IS legal in that they can obtain that which is legally given.
However, the next thought is whether the inhabitants even had the ability to give land that they themselves claimed was not theirs to give...OR, in other instances had been claimed by them without giving the other parties or inhabitants the process to object or deny such claim?
The Utah territory could be seen as both claimed by parties by force or by withholding the ability of objection to the parties that controlled that land, OR, later, by treaty to those that were under ignorance or force, OR, later...by treaty between those that claimed these lands in giving it to the US (by force one could claim).
On the otherhand, when ever land is ceded by treaty ending a conflict or war, that treaty is in most instances seen as binding. A question not asked by you, but could be raised, is if the original parties ceding the land never truly legally obtained it by treaty but instead by claim and force...could they legally cede that land away again to another party that forced them to do so by treaty?
I would not say it is so much as class that gave them the ideas that they legally had hold of the land, but more of force and the ability to enforce treaties of claim.