Because the center of the filter where any contamination would likely end up (i.e. foil or plastic bits from the oil container....which is where this all started) is the "already filtered side". Therefore, anything that ends up there, gets pumped through the engine. The biggest issue reported was bits of foil from oil containers plugging the small oil "jets" that cool the pistons. A tiny plug here will burn up a piston.
I've seen documents from Cummins and or Ram stating both pre-fill, and to not pre-fill. I'm not sure of the most recent procedure.
I saw some place where they recommend filling the filter from the outside so the oil is filtered as it moves to the center. That would catch any aluminum. , I would think the most like place to pick up contamination is while installing it.