To clarify for those coming later: The negative income tax is the idea that beneath a certain amount of income, you actually are given money by the government. If you're over the amount, you get nothing, and in fact are taxed a portion of your income. Depending on how far beneath the amount you are, you might get more, or less.
But that system leaves some people in a bad spot:
* Sporadic work? Now you have to wait until after you should've gotten your money to get the money to make up for it. Without a safety net(which you almost certainly do not have), you're behind on your bills already.
* The above also applies to if you hate your job.
* You're disincentivized from working if the amount you can make is beneath the amount that would give you the negative income bracket.
* Politically, it's a lot easier when everyone can get the benefit. Less claims about abusing the system if it gives everyone the same thing.
* As an employer, giving someone just 4-6 hours a week of work isn't bad unless the employee wants something different. With a NIT it might just not be worth it to the employee.