I have no idea of what to expect the political game mechanics will be so this is purely hypothetical. The subject at hand is different ramifications of planetary bombardment vs. attacking civilian ships of a given empire. I hope it will depend on a number of factors and affect each empire in varying degrees and weighted appropriately.
Each of the AI empires should be weighing these questions among others:
Do I know the existence of the warring parties?
Do I trade with either of the factions?
What commodities are affected on the planet attacked and does the trade interruption affect me?
How far away from my borders is the transgression?
Did the aggressor in the action start the war?
How many commodities are affecting me by the attack of civilian transport?
These are just a few variables off the top of my head. Remember this is just a post to pass the time as we wait for the release. I am in no way suggesting this is how these game mechanics will or should function. I am anxious to see how the developer has approached this.