Nor did anyone say that they did. Please spend some time on the FairTax website to determine that for yourself instead of just guesing about it - or taking the misinformation put out by FairTax opponents who, after all, have their own ax to grind.
The FairTaxers do, however, calculate sales tax in a manner inconsistent with the way everyone else does it. Why? Because the number comes out smaller. It's marketing.