It does depend on your state, but whenever it happens, it’s a news story simply because it doesn’t happen often. Nuance is important:

> Phillips, Debra Harrell’s attorney, confirmed there is no age at which a child can be left on his or her own specified in South Carolina law. The challenge for the prosecutor will be to prove that this child’s needs and care were not adequately arranged before she was left at the park, he said.

There is no law against it (in South Carolina at least). The law they are using for the charge is reckless endangerment. And usually at the end of it, prosecutors drop all charges due to lack of evidence.

Have you ever been subject of a criminal complaint? Not long ago I was detained on some absurd hearsay. I was cuffed, I was fingerprinted, I was strip searched. Tossed in a cell. Made to perform bodily functions in front of officers. You don't know who is going to take care of your pets, your child(ren). You don't know if you'll be fired from your job. You don't know when you'll be released. In the case of something involving a child, it will be used as prejudicial information in the DCS case and the followup investigation. You quite likely will be put on a civil list of child abusers, which makes it difficult to work with children or adopt.

All these things make public playgrounds just extremely inconvenient. It's such a treacherous hazard I can't in good conscious consider it as anything close to a substitute.

Again that’s not how it works. First, whenever this happens it is broadcast widely in the news. Second, there is some debate, a mention that no state law (and definitely no federal law, as the original comment suggested) actually exists prohibiting kids from being at the playground alone, a vague weak law is used and then the prosecutor goes WTF were the police thinking.