Just to add some more information to this thread. I am a Nationally Certified Child Passenger Safety Technician. It's what I did for 5 years as a volunteer and now, finally, get paid to do! I am passionate about keeping kids safe while riding in vehicles.
For those that are questioning the reasoning on this issue or the relative comfort of the child, please remember this...
The recommendations do not come out of thin air. The AAP and other organizations study trauma and fatality reports to see what some of the main causes of the traumas or fatalities are. There are some great research studies out there if you care to wade through them. Then from the data gathered from those reports and studies, they hand down best practice recommendations. Here is a great place to start...
Keeping Kids Safe During Crashes
Now I'd like everyone here to remember one thing, best practices are just that, best practice recommendations. Unfortunately it is up to the parent/caregiver to make the ultimate choice in the safety of the child -- legal or not, best practice or not. There are also real world circumstances that we can't see on a message board.
Please remember that the kids can't always speak for themselves on the level of safety they want. It's the parent/caregiver's job to make that decision... hopefully well researched and informed.