Let's turn this around.
Why do you think states like Illinois and North Carolina added the provision that failure to adhere to the child restraint laws cannot be used as evidence of negligence?
In fact, if you look at Illinois child restraint laws, it clearly states the following:
"...Any person who transports the child of another shall not be in violation of this Section unless a child restraint system was provided by the parent or legal guardian but not used to transport the child..."
If you're transporting a child whom is not yours and the parent didn't give you a child restraint, you can't be cited for a violation of their laws. How about that?
Why did these states add these provisions to their laws?
They are two different issues that have a common nexus in the issue of driving and child restraint.