Well, El Gimpo, crack addicts aren't as stupid as you might think. Sure, they're addicted to drugs and sell handjobs to make extra money, but it takes a special kind of person to be a successful addict.
The main goal of the addict is to stay high. Now, drugs cost money, and for a homeless motherfucker lying in an alley, money is usually a problem. Eating their own feces is a cost-cutting measure, just like drinking their piss.
You see, a person's body processes the drug, and deposits all the drug that you ingest/inject in your bodily waste products. By consuming their waste, crack addicts can reclaim those drugs, thus making a single hit stretch for a few days.