So let me present an example here (quite far fetched, but still roughly the same thing):
The native Americans, which in some tribes are recognized as domestic dependent nations by the us government. Say some of them
decided that they wanted to be completely self governed, get their own governments, military, the works. And they asked Canada, Russia or whoever
for help in doing this.
You would be totally fine with this then, because in essence, it's their land. And you know, they have the right for it.
Or Scotland decided they have had enough and wanted to split from the rest of England and go back to be a nation in their own right.
Or Wales, or a united Ireland, you would think that that was all good and proper?
As mentioned it's silly notion but you get the idea.
The Crimean Peninsula may be self governed, but they are still part of Ukraine, and as far as I've heard, the Ukrainian government hasn't been doing
any sort of horrible deeds to the population, I would have had no problem with Russia if all they would have done was secure their Naval base. But occupying
the entire Crimean Peninsula is going to far. And let's not forget, even if the majority of the population is Russian, not everyone is.