This isn't really clarifying the question of "who acquired the land for them", just acknowledging they (sometimes? often?) had to defend for themselves out there. In the end those settlers purchased deeds to those lands often for very cheap from the US federal government and expected for the federal government to support their Westward movement (which often did happen as well).
They acquired the land from the government and expected some amount of protection provided by the federal government as well.
And even then a ton of that settlement happened after many wars and what not with Native American tribes and groups in those territories.
Without getting into the (worthwhile) details of US expansion, it's important to note that the security provided by the continental and federal governments did not stop raids, and on a scale of services provided relative to dysfunctional present day governments was truly hands-off. There is running water in Aleppo.
They acquired the land from the government and expected some amount of protection provided by the federal government as well.
And even then a ton of that settlement happened after many wars and what not with Native American tribes and groups in those territories.