But it's already less than zero. There's already a list of refundable tax credits.
A flat tax with an exemption of $20K or $50K will either be zero or more than zero if the poor person earns more than $20K or $50K.
The "Fair Tax" at least one form of which proposes a "prebate" is an interesting idea but I'm not really convinced that it's any different than what we've got now, which is a confusing pile of refundable tax credits including child tax credit, earned income credit, making work pay credit, and a handful of other credits which actually result in people collecting money from the government. If we got rid of all that crap then sure, a prebate would be simpler. But what's the difference between giving someone a cash handout and calling it a prebate vs calling it an earned income credit?
For the record, I favor a flat tax with an exemption of the first $X of income. I just can't see a way to get there, because ultimately you can't get around the FACT that a 20% flat tax is a huge tax cut for high income earners who have 39.6% marginal rates ... indeed, it's a tax cut for anyone earning over $100K. (Total fed income tax owed on $100K for 2013 is $21,293.) For this to be revenue neutral or positive, the very large number of people earning under $100K will have to pay more.
You can't spin that as anything other than a tax cut for high earners and a tax hike for low earners, because that's what it is. Whether or not that's "fair" or a good idea is another issue, but it is what it is, and it's unsellable to the voting public and the people they elect.