Both the height and weight of the child is relevant.
Seatbelts are designed to restrain the body in the most appropriate ie risk free parts of the body ie where they are least likely to cause damage and keep the person/child in place whilst absorbing a large proportion of the impact.
So, the correct positioning of the seatbelt across the body is very important. So is the correct style of seatbelts. Until the head to body ratio evens out, the child is at more risk of sliding out of the seatbelt and/or receiving a severe spinal/head injury.
So, in very simple terms - you wouldnt have someone bungee jump with the bungee tied around their neck, or their wrists, would you? However ankles, waist and upper chest are all good places to secure the harness.
This is why there is the point of forward/rear facing, and also 5 point/3 point seatbelts.
There is only so much car, and car seat manufacturers can do to make car journeys safe. Its not alot to ask to put your child in the correct seat and secure them, is it?
Well, apparently so. Hence the law taking control......im all for it.