PhilFish: "What makes one tax rate wrong, and another right?
take Romney's effective at 15% (-ish) vs one of the 47 million whose effective is 0."
What is right or wrong is a moral issue and you might look to your sense of humanity or religion, if you have one, for your answer.
What makes it legal is the 16th Amendment which says
"The Congress shall have power to lay and collect taxes on incomes, from whatever source derived, without apportionment among the several States, and without regard to any census or enumeration."
The phrase "without apportionment among the several States, and without regard to any census or enumeration" has and interesting history as explained by "The US Constitution Online" -
"apportion v. to distribute proportionately Source: NMW
In the context of the Constitution, apportionment means that each state gets a number appropriate to its population. For example, Representatives are apportioned among the states, with the most populous getting the greater share. Direct taxes (of which there are none today) were to be charged to the states in this manner as well.
The need for apportionment of taxes, and the reason for it, is difficult for us to imagine today, but there were good reasons for it. The following is an explanation of the need for the Direct Tax Apportionment clause. It was written by Supreme Court Justice Paterson in Hylton v US (3 US 171 [1796]):
The constitution declares, that a capitation tax is a direct tax; and both in theory and practice, a tax on land is deemed to be a direct tax... The provision was made in favor of the southern states; they possessed a large number of slaves; they had extensive tracts of territory, thinly settled, and not very productive. A majority of the states had but few slaves, and several of them a limited territory, well settled, and in a high state of cultivation. The southern states, if no provision had been introduced in the constitution, would have been wholly at the mercy of the other states. Congress in such case, might tax slaves, at discretion or arbitrarily, and land in every part of the Union, after the same rate or measure: so much a head, in the first instance, and so much an acre, in the second. To guard them against imposition, in these particulars, was the reason of introducing the clause in the constitution."
So it seems we have evolved from protecting the slave owners from having to pay "unfair" taxes on the slaves they owned to the slaves of poverty and low wages from having to pay "unfair" taxes on themselves.
