Why does this job need to be given only to homeless people? If they are paid less than a normal worker would be (and they are, since they aren't paid anything at all), I'd say this is profiteering off the poor.
Because they have the least chance to make money.
(Except the ones in Laguna Beach, CA which can make upwords of $100 a day just begging to tourists, which they throw away on alcohol nightly. The $100, not the tourists.)