Where your comparison with the deer population falls apart is that unlike with the seals, there are no natural predators remaining to perform that population control. Additionally, "natural" population control is where predators, food scarcity, population movement, and disease do the work. Human-driven population control typically consists of "this animal annoys me, so I will kill it whenever I interact with it." As you stated, we are a species of extremes. Open the door a bit on white sharks and every yahoo will want a jaw on his wall; open the door a bit on seals and fishermen will shoot them on sight.