Infant not using own seat? Have you seen the 200# infant video? In the "unlikely event" of an accident/incident the 20# infant times 10g crash = 200# and not even the biggest heavyweight weight lifter is going to be able to hold onto the kid without folding, spindling and mutilating him.
A couple years ago FAA did the sums on requiring all childern to have their own seats and own child safety seats on airliners. The result of the analysis was that it was better overall to not require this becaue of the number of people who would then make their journeys by automobile, a considerably less safe means of travel, and the likely number of killed and injured that could be expected to result. The usual unelected watchdog groups who appoint themselves to watch over us cried foul, heartless animals, etc but as the man said the figures don't lie.
Since many people are economicly driven they weigh the odds and don't buy an extra seat on "Cheapy Jet" to use a child safety seat. Odds are they will be right, but think of the downside risk