You could bet you that those four people would have the same thing happen to them in Japan's walkable cities as well. It is nothing to do with walkable cities.
at least have enough courage to admit that 4 teenagers would make you nervous. there's nothing wrong with being alert. teenagers can be dangerous and unpredictable. the dividing line IMO, would be behaving preemptively in such a scenario.
4 teenagers would not make me nervous. Happens many times per day for many of us. Parent comments are talking about something different than "common sense" or "being alert". Profiling is a real thing that many people have to deal with.