I don't know where I read this, but there's also a related safety reason. Driving, even with proper car seats, is still more dangerous by magnitudes than flying with an un-belted lap infant. If someone were to choose a long drive over a flight because they'd have to pay for a separate child seat, the child would be at increased risk, despite the fact that flying as a lap infant is *quite* dangerous.

But I agree with the basic premise: if at all financially possible, get a seat for the child, and bring a basic car seat along. We think that we can hold on to them during turbulence, but the reality is that we can't always.