Re: "Aren't they then driving without insurance?"
"It's a useful arrangement to allow you to get out of a logistical fix, but if you pang it, you're liable for the full cost of the damage to the car."
It's a pretty small risk, albeit with a potentially high cost, but most people don't actually have accidents in cars. One or two in a lifetime maybe, but it's not, for the vast majority of people, a frequent enough occurrence to be worry about.