Two anecdotes.

Perfect Dark, the N64 game released around the year 2000, got given an 18 certificate in the UK because of the fact that blood can splatter from shot enemies onto walls.

Raiders Of The Lost Ark, the film released in 1981, nearly got refused a PG [edit: thanks, bena] certificate because it showed exploding heads.

The picture you linked depicts both of these acts of violence, whereas Wolfenstein 3D does not. Maybe Apple have similarly esoteric criteria for unacceptable acts of violence as the film and video game rating boards.