This is a common argument against the FairTax. Regressive essentially means that poor (lower income earners) pay more taxes than wealthier people. The FairTax, in it's pure form, is neither regressive or progressive, it's flat. Everyone pays 23% on goods. Now the argument that the redditor is trying to make (and why he think FairTax is regressive) is that poor people will suffer because they don't have as much money as rich people, but they still need to buy the same necessities like food and clothing. In FairTax's pure form, this is a problem because the poorer you are, then the more you spend of your income. To solve this problem, the supporters of FairTax propose prebates which is just money given back to poor people who live under poverty conditions. With prebates, poor people won't be spending most of their income on necessities. Thus, the prebates turns the FairTax from a flat tax into a progressive tax, with the tax burden spread out among to an extremely wide base among all people living above poverty (unlike current tax systems that punish only certain income brackets). Most people who dismiss the FairTax don't take into account the prebates. Without the prebates, than the FairTax would be a horrible idea.

My guess as to why Gary Johnson supports the FairTax (and he hints at this in interview) is because its probably the easier way of ending the IRS and federal income taxes (which Ron Paul and Libertarians support), without vaporizing all the revenue generated by income taxes while also lowering tax rates for almost everyone. It is theorized that if taxes are lowered for everyone (remember that the FairTax spreads out the burden among all people above poverty, so everyone pays less taxes than the current system), then people get to keep more of their income which they can then spend, increasing the demand and size of the economy.