Note: I am pretty passionate about this particular topic, and many topics, and realise this can come across as aggression. I just really, really love discussion, and challenging my own attitudes through talking to people I disagree with. None of the below is intended aggressively, I'm just enthusiastic to discuss it, all is in good faith!
How many of the people here referring to what is and isn't good parenting are parents themselves, or have ever been responsible for the care of teenagers?
I don't think you understand how negligence works in this context, or at the very least, how the law tends to say it works, regardless of whether you share the law's opinion.
The point here is not 'when I was a kid I was left unsupervised and didn't die, nowadays kids need to be wrapped in bubblewrap.' The point here is that you were lucky. And 99.9% of people are lucky. And 0.1% of people are the 14 year old kid, left unsupervised by a 'responsible' adult, be that a teacher, a friend, or a parent, who does get fucking killed by one of these contraptions. That is what the law is there to prevent, because not a single one of those should ever happen.
One thing we do agree on is that it is impossible to keep your kid safe at all times, but there is definitely a middle ground between bubblewrap and not being left in a position where you could have done permanent damage.
This is a forum where attitudes and opinions are frequently challenged, and I am frankly baffled that I'm the only person so far arguing in against leaving literal children unsupervised with power tools simply because you guys managed to lose just a nail, rather than a finger or a hand, and presumably your tendon repaired itself, rather than costing you sensation or dexterity in that hand.