Bullethead, your argument isn't logical. You say that there should be a PNG system so that if you shoot your own side you'll be punished for it, but you also say the main problem is not the shooter, but those who jump in with E and steal the kill. So if it's not the shooter's fault why should he be punished with PNG?
The bottom line is that the great majority of the time friendly fire is the shooter's fault. Getting pinged by a friendly when I'm lined up on a target is the last thing I need. More often than not the pings won't be enough to kill me, but may make me break off my attack, so the shooter is not punished at all but I become a victim of his carelessness. Plus if I end up shooting myself because some dweeb jumps in front of me that seems a much less harsher punishment than going PNG for 24 hours or whatever. Hence killshooter is the best solution to a tricky issue.