The thing about all the "was/wasn't it normal" speculation is that it actually doesn't matter at all what you personally experienced as normal because we tend to reflect the majority view of our environment and peers. So it tends to seem to people as though what they personally consider to be normal is universal, but this is not usually the case, especially for things where the predominant view has changed or is in the process of changing (which, I think, leaving children unattended has undergone a huge cultural shift between say 1970 and today).
Internet forums were well underway in 2005 and for a few years before that. Mumsnet itself has posts going back to 2000. You can freely go back and see for yourself that it was absolutely considered normal by some people, which does not mean that every single person who had children around that time would have considered it a reasonable decision to make.
Behavioural and cultural norms (e.g. which risks are considered appropriate to take) take time to change, and won't flip in an instant for the entire country overnight. It is just as correct for one person to say "In the 80s, nobody used car seats for children" as it is for somebody else to say "In the 80s it was absolutely normal to use car seats, they were sold in Mothercare and everyone I know used them".
And regardless of whether leaving children unattended is considered normal/reasonable, I think absolutely everyone would agree that it is NOT a normal response, on learning that a child is unattended, to abduct them. The normal response would be (variously) - ignore, try to find the parents, contact the police.
Stranger abduction is exceedingly rare specifically because of this norm. Most people do not want to hurt children. Most people, even those who would hurt children do not abduct children. The issue is in this case, police are now aware that an extremely twisted, sick individual already found guilty of other horrific crimes of a rare and related nature was in fact highly likely to have been in the area at the time of the disappearance. So while stranger abduction is rare generally, because the kind of person who would abduct a stranger is, thankfully, rare, the odds of that being the answer increase enormously when you know that such a person was in the area.
It's like if a person had a mystery injury which looked like a spider bite, saying that a certain kind of spider bite is rare because that species of spider is rare, so an injury could not possibly be that bite, but then discovering that there was one of those spiders in the room where the person was bitten. The premise that this spider is very rare no longer matters, it unfortunately becomes extremely likely that the bite was in fact from that spider.