While this is true, the source of genetic deficiency is not as big as it seems. People end up screwed up in the head due to life choices more often rather than the opposite. I have been around every type of human in my short lifetime, and met only a handful of mentally retarded cases that are homeless. Even those are intermittent, as medications level them out and/or they do not want to be without a dwelling. In contrast, the homeless that are "permanent residents of the street" all are with it, at least enough to go and seek help from state/fed/local resources. They just don't, often due to UAs.
This is a touchy subject when it comes to human rights, and no one is to say who is competent or not. The USSR was looked down upon due to institutionalizing incompetents, but the society as a whole benefited. Think of kittens, they don't even reach the teat if they have issues, the mother cat will kill them. I'm not saying leave them for dead, that's cruel, but giving the people the "rights" that know not what to do with them is just the same. There are ways to involve and employ people that can not survive when left to their devices, what Goodwill does is an example.
No one just gives a rip enough, and the ones that do let their emotions overtake rational decision making.