See: http://www.faa.gov/passengers/fly_children/
The problem (politics) of mandating child restraint systems for air travel is the cost of buying a seat for the child. Too many parents are either trying to economize or really can't afford air travel if they have to buy that seat. From an airline point of view, a seat is a seat in terms of cost and revenue with the slightly lesser weight of a child having a negligible savings on fuel. (They don't charge adults weight over 200 pounds extra so why would the discount children weighing under a 100 pounds).
The FAA is apparently reluctant to mandate restraint systems (and the separate seat needed to accommodate the restraint) ironically from a safety point of view. The FAA apparently has been concerned that to the extent air travel becomes too expensive for family travel, the family (with the small child) may elect to drive to their destination rather than fly. Statistically speaking, a child is safer flying without a separate seat and qualified restraint system then driving with a qualified restraint system.\
I suppose an alternative would be a Fed regulation that mandates separate seats with restraint systems for children AND mandates that airlines significantly discount those seats. The airlines would then have to up the charges for adult passengers to offset the loss revenue. I'm sure many forum participants would have something to say about higher fares.
PS. I'm one of the choir. I bought a seat for my first child then about 9 mo. of age and tried to use our car seat restraint for that purchased airline seat. I was not allowed to use the child car seat and told to hold the baby during take off and landings. We did get to use that purchased seat during flight so our son didn't have to kick a non-family passenger.