That's the part I hate, which is what I quoted from the article in the original post I made (that you replied to).
If I make $100 per year and pay $25 in sales taxes that's a 25% effective rate on income.
But if I make $200 per year and pay $25 in sales taxes that's a 12.5 % effective rate on income.
IMO they do that to try to make something look unequal. But the sales tax rate is not related in any way to income. It is based solely on retail sales.
The idea they're trying to get at is to try to say that the sales tax is regressive and they use that as a reason to object to the sales tax. A sales tax can be regressive but the Fair Tax has the rebate thing to minimize that.