The income level has nothing to do with the level of risk of the job and chance of being robbed. If the messanger says that his income is 25,000 pesos a month, he doesn't have to declare it or pay any income tax at all. But if the same time he has to deliver envelopes with thousands of dollars to deposit to the bank on behalf of the office he is working for, his life is in danger because of that. So he can be poor but he still needs a gun. Actually, he has more risk than the owner of the big company siting in his office with 10 armed guards downstairs.
The same is with self–employed photographers, for example, who have to move around the city with the equipment that costs a lot and is a magnet for robbers, but they may make less money than the amount that has to be declared.