They are already programmed to avoid being smashed from behind. Watch these two cases [0:47, 1:54]
https://youtu.be/JwknRVjUBCg?t=47
https://youtu.be/JwknRVjUBCg?t=114
Why would Autopilot have a warning for frontal collisions but not for rear collisions? And why would Autopilot be speed-limited and all that if it was able to use launch mode to avoid a rear collision?