Ah, statistics. The issue here is one of "statistical significance". As a generality, anything under .5% is going to be considered statistically insignificant, and the fatality numbers for commercial air passengers in the developed world are well under that threshhold, as a percentage of the number of enplaned passengers total. Any time you want to pass a law requiring the use of a safety device that costs money, you have to prove that the protection provided by the device is statistically significant for the population at large. Unfortunately, the FAA is taking the position that the risk at issue is that of accidental death, and there is just no way to prove that the benefit of carseat use on a commercial aircraft is statistically significant in that respect.
The American Academy of Pediatrics published quite a few statistics in favor of carseat use in their position paper on this topic.
http://aappolicy.aappublications.org/cgi/content/full/pediatrics;108/5/1218 However, even they conceded that the risk of ANYONE dying on a commercial aircraft in the US fleet is *way* under .5% -- one in 8 million, actually. However, the numbers for injuries requiring medical attention were quite a bit higher, 852 in a 15 yr. period between 1981 and 1997, in 342 reported incidents of turbulence. As the AAP statement shows, a big part of the difficulty in getting good statistics to defend this position is the lack of a requirement that the passenger census include age data. Right now, the only way a kid is counted as a kid on the manifest is if he is flying on a child fare. We all know that at least half the time, a child fare will cost more than an adult sale fare, so most children are not marked as children on the manifests.
So here is my position on this: Lap-carrying is legal, and it is relatively safe. I don't waste my time and energy arguing with or alienating people who deliberately choose not to use a carseat for a lap child, and I would not class those people as recklessly negligent. They considered, and they chose, and they have that right. I would much rather concern myself with helping to get out information to those parents who just haven't considered the safety implications at all, especially for children who are over 2 but weigh less than 40 lbs., as the whole cost angle usually doesn't matter for them at all.
That said, I spend a LOT of time politely urging people to use carseats on aircraft. I very often offer to carry them onto and off of planes for other passengers, and even to install them if the person seems to need help. (FA's tend to be very grateful when I do this, because I've done it a lot, and I'm fast.) IME, families who use carseats just tend to end up more comfortable during the flight in every way, and yes, are better protected in the event that the unthinkable happens. I offer advice here and on other boards about how to cope with transport logistics, and installation tips. I make it a point to keep all advice positive, and never to criticize those who don't decide to take me up on it.
Jodifla and I have been around the block on this one before, and for whatever reason, I've apparently seen a lot more turbulence and near-miss ground incidents than she has. Believe me if you want to or don't; that is up to you. I can tell you that I have seen unrestrained children get injured on flights, sometimes badly enough to need medical attention, though more often, hurt just badly enough to scream for quite a while as their parents wrestled with ice packs and wads of paper napkins and worried themselves sick. Most of the time when your child gets hurt, it is only a matter of minutes before you can get medical advice about how to judge the situation, or to get emergency medical help. At 30,000 ft., there isn't much you can do, and often no help beyond the flight attendant's first aid training. Even if the situation is serious enough to warrant diverting the plane, the process of getting to medical help is agonizingly slow. My feeling is, if using a carseat will make it even a little less likely that I'll ever have to deal with that scenario, then it makes sense to do it. YMMV.