Irishguy28's answer is correct. What you think is a loophole is in fact your making yourself pay for something you should get for free, without getting any additional advantage whatsoever by paying for it instead. All airlines charge increasing amounts for further bags because having passengers with 3, 4, or 5 bags is both niche and a logistical risk (of going over capacity on a full flight) whilst the first bag is considered an essential by many.