The US is a settler-colony, that's an entirely different situation. The Donbass region is historically a group occupied by ethnic Russians, Russian is the dominant language, and it has been this way for centuries. Even a basic Wikipedia article makes this clear. Whether or not the Donbass region is historically occupied by ethnic Russians isn't what's in question, what's important is that the post-Euromaidan government is a group of nationalist Banderites that have been suppressing ethnic Russians, including erasure of the Russian language.
This is why it's important to recognize that the seperatists have very good reasons to want to leave Ukraine. Those that are against their ability to join Russia through annexation must make the argument that they not be allowed to secede even from a government that has been shelling and slaughtering them.