I think you have to understand that posts like this reflect the attitude of Americans much more than they reflect the USA itself (I grew up in the UK but live in the USA).

More concretely: are there places in big cities in the USA where you should be very cautious walking around at night? Absolutely. But the exact same thing applies in London and Manchester too. Robbery is an issue there too.

To some extent, people in the USA just make a bigger deal out of it. You'll also find that when Americans discuss travel abroad they're also very worried about safety. IMO it's a cultural mentality more than the specific realities of specific places.