Fire arrows griefing the map (TNT)

Even with "/gamerule mobGriefing false" and "/gamerule doFireTick false", skeletons and wither skeletons with Flame I enchantment on their bows (wither skeletons have fire arrows by default even without enchantments) are able to set off TNT blocks if they happen to hit one. This is direct griefing of the map by a mob, which should be forbidden by the gamerule.