Mental illness is different because in many cases, the people in question are do not possess the faculties required to live normally, decently, or (in rare cases, peacefully) in society. They often end up in jail. A proud person's only choice here is institutionalization of some sort. The alternative is the status quo: the able suffer injuries to their own rights, the mentally ill suffer indignities they had no hand in creating, and the able end up paying just as much to keep them in jail anyway.
In a sense, I think the reason someone is poor is more important than the fact that they're poor at some given moment in time. I believe that a just system concerns itself with making it possible for those who desire more wealth and are able to earn it to do so, no matter what their present level of wealth is. For the remaining handful of people that don't fit into that category (and don't suffer from a grave mental illness)? Hey, no one is stopping you from helping them.
In a sense, I think the reason someone is poor is more important than the fact that they're poor at some given moment in time. I believe that a just system concerns itself with making it possible for those who desire more wealth and are able to earn it to do so, no matter what their present level of wealth is. For the remaining handful of people that don't fit into that category (and don't suffer from a grave mental illness)? Hey, no one is stopping you from helping them.