Because letting a very maneuverable PT boat get rammed by a cumbersome destroyer is the naval equivalent of letting your car drive over the side of a bridge.
The fact remains that there was only a single ship commander that was court martialed as a result of the loss of his ship in combat. Of all the ships that were sunk, only one. Yet, many here would have had Kennedy court martialed for the loss of a PT boat of all things. Should all of these CO's have been court martialed as well.
The fact also remains that what Lt. Kennedy did in the aftermath of the sinking of the 109 was undisputably heroic. How many of those who are so quick to slander him could have done the same thing?