"Honestly, I think there are very few parents out there who think ‘my child isn’t that important to me, I don’t really mind if they get abducted.’
But people do assess the risk differently."
I also think some people approach the issue in a more simplistic / nuanced manner when approaching this debate.
For some parents, it is a simple case of my child might get abducted, that's irresponsible.
Others might take a more nuanced approach. You are more likely to win the lottery than to have your child abducted by a stranger. Even within that very rare risk, it is even rarer for an abductor to forcibly take a child. Most will simply try to abduct another child instead (Eg the April Jones case). The child saying "no" is their best protection against being abducted.
Your best bet to save a child from abduction is not to watch them 100% of the time, it only takes a moments lapse of attention (think Jamie Bulger) but rather to teach the child not to go off with strangers. Of course, that requires the child to have some degree of freedom to be able to do this.
Others may also look to other countries to see when children are able to take on this level of responsibility. Perhaps they're aware of the many countries in continental Europe where children can walk themselves a mile to school alone, unsupervised every day, or Japan where 5 year olds navigate Tokyo's metro system alone every day to get to school. This shows that children are capable of being unsupervised from the age of 8/9.
Perhaps the parents are also aware of the psychological damage we're doing to our children's mental health and social development by not allowing their children to have unsupervised play. They might have read the studies, the research and the concerns by numerous experts about this, as well as the studies that show we have the least independent and unhappiest children in Europe.
So whilst some parents might do a knee jerk no it's not safe, others might take a more nuanced approach, balancing the unlikely but devastating harm from abduction against the more insipid but more likely mental harm we inflict on our children by not giving them freedom. It's all a sliding scale, and what some find acceptable, others will not because they will be considering a different range of factors in their decision making.