The problem is that any tax will by its nature influence the people to spend or save their money one way or another. Even if an agenda isn't being advanced (fairness is an agenda too), one will be felt.
Q, But you're wrong. A true flat tax based on income, from whatever source derived, with no deductions, and/or exemptions does not do that. It gives equal economic opportunity to all. The income tax codes of today are written with "fairness" in mind with the graduated aspect, and with social engineering in mind through the deduction and exemptions allowed. A true flat tax would put an end to that. In Maryland today, more than one third of "income" is not taxed {The numbers are in the billions} due to deductions and exemptions given by politicians on any given day to those who ask and/or demand "fairness". Meanwhile, those who pay taxes on the remaining two thirds must pay a higher rate of taxation in order to make up for the "loss" of revenue due to deductions and exemptions. Make ALL pay the same rate on income from whatever source derived. Why for instance, should millions {billions?} of dollars of income made by interest and/or stock dividends not be taxed at the same rate as the income of a ditchdigger? No deductions and/or exemptions. Peace and love, George.