The person was near enough to spot the child and take a photograph, so they could have remained at that distance and call the police if they feel it's necessary, not least to advise the returning parent/guardian that the police were called. In that case the police would probably have arrived and written a ticket or something. Although this would have involved a delay and frustration for the temporarily unattentive parent, it would have been better than the actual outcome involving the issuance of arrest warrants.
One could say 'why not let the matter drop, the Mom came back and so everything was OK' but that's a classic example of hindsight bias. From the police POV, a child was left alone in a vehicle that was later driven off by someone else. When they followed up, it turned out that the person who drove off was not the owner of the vehicle (which belonged to the child's grandparents). They have to at least consider the possibility of malfeasance.
«From the police POV, a child was left alone in a vehicle that was later driven off by someone else».
No "someone else", but the very same woman: leaving the car, returning back to the car. The same woman, not someone else. Unless the good samaritan has deceptively shown just the last part and told the police he (or she) saw a different person leaving the car with the child inside.
The issue here is not car theft, driving license not matching the car owner, or whatever. It was instead a good samaritan evaluating as criminal/dangerous the fact that a mom left his son alone in the car for few minutes. And decided (when she came back) to call the police to...? Check if the child was fine? Whatever.
It takes few minutes to check if the car was stolen and/or the owner knows that someone else is driving it.
Your firs para: could you imagine the situation where you tell a parent that you called the police because you were worried about their child? That's not an easy conversation.
I agree that people should report early and report often; and allow experts to make decisions. Sadly, reporting early and often just swamps people who are already far too busy dealing with children who are being abused, or it dumps reports with police who take action when they don't need to do anything more than have a chat.
I agree it's not an easy conversation, but if you think it's important enough to take photographs and call the police about it was important enough to communicate to the person who you have made a potential suspect. In this case it would probably have resulted in a much less traumatic outcome for the author of the story, who at worst might have missed her plane.
One could say 'why not let the matter drop, the Mom came back and so everything was OK' but that's a classic example of hindsight bias. From the police POV, a child was left alone in a vehicle that was later driven off by someone else. When they followed up, it turned out that the person who drove off was not the owner of the vehicle (which belonged to the child's grandparents). They have to at least consider the possibility of malfeasance.