I saw a number of interesting and justified but also the common silly answer to the topic here.
One thing should know everybody who is dealing with the topic. Airmen forced to land on the enemy side of the lines are trying to either burn/destroy the airplane or to fix it (if possible) and to restart. I have seen many reports showing that Allied airmen did burn their airplanes if German troops were not fast enough to capture it. As well there were also some cases with enemy airmen who did quick repairs on German-hold area and re-started and escaped.
So Germans aviators expected that enemy airmen did leave their airplane immediately after an emergency landing and did keep some distance to the airplane. As well they should show their surrender (waving etc.) in some way - otherwise MG fire ahead of or into the grounded enemy airplane was seen as justfied to prevent the afore mentioned destruction or tries to re-start!
Adolf Ritter von Tutschek reported one or two time he forced enemy pilots out of their planes with MG fire and prevented the destruction of these airplanes by their pilots.
This approach becomes problematic if the enemy aircrew or pilot is severly wounded and unable to leave the airplane without help. Nevertheless the victorious attacker can evaluate this kind of situation.
Finally we have to consider that the group of "haters" among the aviators - on both sides - was also willing to kill helpless enemy aviators.