They're not specifically feeding them plastic only, he's showing how any waste food (old potatos, old bread, chips, etc) that comes to be processed does not have the plastic bags removed before going into a grinder to become feed.

He actually shows how it's explicitly allowed in his state regulations, likely approved due to the efforts of an agribusiness lobbyist. I suspect that any negative effects to the pigs is not enough to effect the end product/bottom line, or doesn't manifest within the timeframe of a viable animal for slaughter.