I was inspired by that song by City High - remember a few years ago they had a single called "What Would You Do?" I think the chorus went something like "What would you do if your son was at home, crying all alone on the bedroom floor, 'cos he's hungry and the only thing you can do is sleep with a man for a little bit of money..."
Suppose someone seriously felt as though they were at a dead end and that was their only way to make a livng?
It definitely is complicated, I would like to see everyone who works as whatever to get taxed, but as Ushgarak brought up earlier: drugs. Don't want any drugs flying around, dealers getting taxed?!
I think test-eating pet foods is sick.. But some people still do it as a job. Yuck.