I dont think it cruel at all.... These are boy scouts, not cub scouts. Everyone does not need to be a winner. If it has been tradition to reward those scouts that complete the orienteering course with ice cream, then by all means they should get to enjoy it. If the ice cream reward is that important to some boys, maybe next time those boys who did not attend will.
If those with the ice cream were flaunting it at those without, that could be a cause to stop and have a discussion.
If this were Cub Scouts, I MIGHT agree with everyone getting ice cream or no one. But Boy Scout age b