Of course you are impugning other people's moral and ethical judgment when you say that they are valuing money and their own selfish desires rather than purchasing a seat on the plane and lugging a car seat around with them.

I believe seat belts are required on planes for multifaceted safety reasons: to prevent injury to self, to prevent injury to others, and on a macro level to control a large number of people jammed into a small space. Infants are smaller and more easily controlled by a parent than toddlers and older children. In the vast, vast majority of flights, they are safe in their parents arms and leave the flight uninjured. Because infants are light, relatively speaking, they are also less likely to cause injury to others than adults and larger children are if not belted in. Finally, an infant in the arms of a parent who is belted in is as effectively immobilized as the other people on the plane. The seat belt signs enforce people sitting in their seats and staying there, and seat belts help people comply with flight crew directions and stay put.

I'd think a rational argument could be made that airline lap infant guidelines should be based on the child's weight rather than age. And who knows, maybe we'll see weight-based pricing for everyone someday.