For some homeless taking drugs is done to not die of other homelessness associated cause of death. When your on the street with no where safe and warm to sleep at night you may turn to drugs to keep you up and moving so you dont fall asleep and freeze to death or fall asleep and get mugged/raped/stabbed.
They may be desperate to hire anybody, but not everyone is willing, with good reason, to work 16 hour days with no overtime pay and sub-minimum wage no benefits and work for only part of the year and get paid under the table. only reason it make since for migrants is the low cost/standard of living back home and currency arbitrage where us dollar are worth so much more than their own currency.