Because that would make Vader evil. Won’t someone please think about the children? We can’t have millions of kids idolizing Han unless he didn’t shoot first, and we can’t have millions of kids idolizing Vader unless he was only temporarily turned to the dark side over his despair about his beloved’s death.
My response of “Code Red: Kermit’s down! Kermit’s down!” nearly got me kicked out of the cheap show where we were watching the movie. But I digress.
I thought this was stupid when I saw the movie too, but I found out afterwards it’s actually continuity. Apparently, the last episode of the “Star Wars: Clone Wars” cartoon (which ended right before the start of Episode III) ended with Mace Windu attacking General Grevious, wherein Mace managed to Force-crush some of Grevious’ “biological components.”
Hence the cough. It’s not a silly gimmick, it’s a fresh-off-the-battlefield wound. I’ll give it a pass.