I took a look on this thread of Axishistory forum:
http://forum.axishistory.com/viewtopic.php?t=68936
My guess is that there will be never a common agreement, if shooting the parachuted pilot was legitimate or "morally acceptable" in WW2. It was only after WW2 when international rules of warfare considered the strafing of parachuting pilot as a war crime (international conventions of 1949 and 1977). Probably the issue of killing parachuting pilots was taken up after WW2 precisely because it was so common during the war.
In this thread it has been stated, that RAF commander Downing during BoB accepted the principle that parachuted pilot is a legitimate target when parachuting over his own territory. Well, my guess is that he would not have liked to see his RAF fighter pilots to be annihilated when parachuting over Britain. Both he and British propaganda would have made a lot of noise about the cruelty of German airmen! I think that there has been some hypocrisy on this issue based on double standards. "Our pilots" can strafe parachuted enemy pilots to eliminate them from further combat, but if the enemy does the same, they are "criminals".
I can see Japanese practice having been consistent (eliminate the enemy from further combat whenever possible and kill him when parachuting over his own territory). Some pilots may have enjoyed of attacking easy targets, but for most Japanese pilots it was probably following orders which had logical basis.
At the same time I understand that parachuted and attacked Allied pilots did find it repugnant. I guess that all of us would have felt it that way in their position - you donīt want be strafed while parachuting and without any means to defend yourself - you would very likely feel that "it is not fair".
It is a question on whose position you find it easier to identify.