I wanted to create a zombie wearing armour in my creative mode testing world, in which I had mobGriefing set to false. I understand why the gamerule would prevent zombies from picking up items from the ground, but I did not expect and do not understand why it prevents the player from explicitly creating a redstone contraption specifically designed to equip mobs with armor.

I believe that the functionality of redstone components such as dispensers should be considered to be entirely a player action, and not a "mob griefing" action; it is completely unlike creeper explosions, enderman building, and enderdragon destruction.