From local laws. From physical laws they are not. As a kid long time ago, I was sitting in a Fiat 124 when a bus driver wasn't paying attention to the road ahead and hit the stationary car on intersection in full city speed, thankfully it was direct hit from behind and there was nothing in front of the car, so there were no serious and only some very light injuries, but it was ugly.

But I cannot image having a kid sitting freely in a lap nowadays just because the car in question is exempt from regulations. There doesn't even have to be a collision, emergency braking (either human or autonomous) is enough to throw the kid off against the car interior and sustain injuries (yep, had that too in the old no-rear-seat-belt-times when an ambulance without siren appeared oncoming wrong way, almost bit my tongue off).

Don't use restraint systems just when they are required by regulations. And especially in nowadays tenfold traffic.

Absolutely, but I think the context was clearly indicating that we were talking about human laws, not physical laws.