Yes they are all signs of desperation. Maybe you could argue they are indicators the person will accept lower wage and as such less expense to the employer.
Yes, that's my point exactly. These are workers with no social safety net, no knowledge of local labour laws, no union membership and a desperate want to not have to go back to where they came from. They are ripe for abuse.
Then why are the homeless not employed?
Because the reason they're homeless to begin with is that they could not find or hold a job. Usually this is because of untreated mental illness and substance abuse problems.