I see all the the things you mentioned as threats, and as you said, some are worse then others. the parking lot being more dangerous then the people in it. but the reality is, child abduction is still a threat, even if its a very very small one. its still possible. and its one of the things that is much less possible if child is in your sight then if child is not in your sight. so that's how I assess risk. while something may not be VERY dangerous, it can still be MORE dangerous (even if only a little) then my other options. KWIM? I agree with you though, the other things you mentioned would be my bigger concerns as well.
I really do think in many cases, walking the child through the parking lot is a greater risk than the other. Especially in the winter. But there are so many more parking lot accidents than strangers who take children out of cars that it seems like if we looked at this objectively it would be obvious which would be safest. The only reason the predator risk seems big is because the few times it does happen, it's on the news for weeks straight, and kids get hit in parking lots often enough that it isn't even a news story so we don't hear about it.