When a government policy can be tricked it will and illegal markets will emerge. Here is great example.
"Tuberculosis sufferers in Khayelitsha, Cape Town, were found to be selling samples of their sputum to healthy people to pass off as their own in a scam to gain medical grants."
Buyers of the samples are "then able to get a card from a clinic indicating they have TB and use this to fraudulently obtain a temporary disability grant of R1,010 per month from the department of social development."
It is not greed, it's human nature.