SF is as bad as I have seen, but many other US cities have scary homeless scenes. No other country in the world that I've been to have I seen this phenomenon. It's similar to no-go zones in American cities. Everyone just pretends it doesn't exist.
It sounds like you're trying to argue for some weird conspiracy theory. There are dangerous areas in US cities, but that's not the same as the (generally mythical) idea of "no-go zones".
It's no conspiracy, it's a racket called "gentrification". The hoods get cleaned up, some naive, young professionals buy in @ inflated prices & then the sun goes down & everything not bolted down is stolen/destroyed. Then there's the break-ins, muggings & murders, but we don't like to talk too much about those aspects.
Conspiracy? What? First day of work I can remember three different people told me "dont ever on go the other side of the highway; dont even drive through it."
First month of uni I unknowingly drove into the wrong neighborhood and stopped at a park. As soon as I turned off the engine and got out a group of people that were playing basketball started pointing at me and running at me. I dont think they wanted to introduce themselves in a friendly way. Later that semester, my friend wondered drunk into that neighborhood and was sucker punched and robbed of his wallet and cell phone.
Funny you call this some conspiracy? I know it's not PC, but conspiracy? Really?