I saw this outside of NYC, but it appeared that most of the trash was tossed down embankments from adjacent roads. I figured that it wasn't due to the homeless but just dumping from people that didn't have garbage service.
What I saw was people pulling up with trash behind office buildings and unloading it in the WM containers. Then the “homeless” come along and collect the junk and spread it all over. Even if you padlock the containers they come with bolt cutters and open them, then to punish you, they spread the trash all over the property. I have seen the RR do all sorts of things to frustrate them, but people being people, they find ways around it.