Ah. That's a good point. Hadn't thought of the angle. Although one could then make the argument that a few of the other planetary improvements should be destroyed as well. Like a few of those 15 Research buildings.........
I do not know if the explanation is true or not but, assuming that it is, unless every cell that can be developed is developed when you assume ownership, how would you know what had been developed and is now destroyed?
This is another example of an improvement the devs can make. If improvements were destroyed during the invasion, why not graphically show the destroyed improvements and give the players the option to clear the destruction and build new improvements on the cell? This should include building a new capitol!