EITC is not a negative income tax. In a negative income tax, below a certain level, each marginal dollar of reduced income produces an additional negative amount of income tax (or, equivalently, positive amount of refundable credit.)
EITC is a work incentive program that provides additional income tax credit with increasing labor income up to a certain amount -- the feature for which it is named as a tax credit for earned income -- and then tails off above a certain point. In the range from the peak to the tail off, it might be mistaken for a negative income tax, but that's true of any tax credit (or even, really, a tax deduction) that has a soft cap. But that's not really the basic design or function is, and, while the EITC may work fairly well, there's no good argument I can see that the negative-income-like portion of the top end of the EITC benefit curve is a contributor to its beneficial functioning, except perhaps if you consider it only against having a sharp cut-off where the benefit drops from the maximum to $0.
