That's a very slippery slope argument, what if that child is bullying someone, you're not their parent so you're going to let it happen?
Or what if the kid is messing around with things that can be harmfull, like entering abandoned houses not your child so not your problem?
What if a drunk 16y/o is getting in to drive his whilst he's intoxicated also not your problem?
or something less drastic what if a kid is being an asshole with the parent letting it, also not your problem?
Or someone who's holding his kid over his head over an animal encounter fence so it can see better, also not your problem?