People do this because they're crackheads (or heroin addicts, or methheads, you get the idea). It's not a poverty issue, it's a drug issue. The person working a minimum wage job and sharing a studio apartment isn't going to dispose of their old bicycle in the river. The person who steals a bike and realizes they can't sell it to get their next fix probably isn't going to have a problem dumping it in a lake or river. They're already leaving needles on the playground, shitting on the sidewalk, and assaulting innocent people for not giving them a cigarette. Do you really think they give a damn about the environmental impact of dumping their stolen goods in a waterway?