Who is to say whether fear is "warranted"? Bad things COULD happen. It's risk averse and may not make cold, logical sense compared to, say, allowing teenagers to go boating on their own, but parenting is driven largely by emotion. There is also the often unstated fear of appearing to be a bad parent. I suppose the real question is whether there is actually a greater risk of something bad happening IF parents let their kids play until dark on their own. I don't know the answer to that but I admit that I will still parent my kids within the rough parameters of socially accepted conduct for my peer group. (My wife and I are actually known among our friends as being less risk averse with most things but we are still "in bounds" even if nearer the boundary than most).
I think this is an actually much more interesting question than the OP's...and I have a take on it.
Despite my earlier comments about evil and whatever, I'm actually a bit of a helicopter parent. This is largely for personal reasons...both because of bad things that happened to close family members that could have been avoided with a little bit of forethought and from an incident with my eldest son when he was 2 where my small bit of negligence could have been disastrous. Because of these, I tend to hover more than I rationally know I need to. Yet in my head, and many times out of my mouth, I'm known to tell him and his brother, "My job is to protect the 30 year old you from the 10 year old you...but I also need to let the 10 year old you be a kid so that the 30 year old you doesn't hate me later."
If I were less rational and did not understand that my fears are born more from my own personal issues and not so much with real world issues, the poor kid would never see the light of day.