The point of the film is to show how horrible war is in a context Americans can relate to. If they made a more realistic alliance, down some sort of real life right / left politics the message would be lost and it would be held up as some sort of propaganda film by one side of politics with the other side using it to justify why they're correct.
So, yes the "alliance between the California and Texas" is a very deliberate choice.
Also the idea of the two most economically independent and arguably most "separatists" US states forming an alliance in a modern civil war is really not the stretch that most Americans with their ideology blinders on might feel it is. Two large polities that wish to be sovereign lean on each other to support their parallel ends? That's actually tenable world-building, I think.
It was a bit much to work with, but once I realized that the civil war itself and the whys weren't what the movie was about, I went with it. This scene was the most disturbing of them all. Maybe because it's not that hard to imagine some people going this far. I'm sure there's some veterans of various conflicts that would agree and saw it happen.