After the demise of the income tax code, you will be able to withdraw any amount of money (principle and/or interest) from any account you wish. There is no tax on income or earnings. End of story.
Under the current income tax system now, whenever you spend any money, you ARE currently paying the embedded taxes for goods or services. It does not matter where you got the money. It could be from your checking, saving, 401k, Roth, sale of stocks etc, you will still pay the embedded tax costs. There may be double or triple taxation involved. The FairTax is a replacement tax on this embedded tax costs just the same as you are currently paying. There is no need for an adjustment for the Roth. You may want to congratulate those that put their funds into the 401k with no income tax payment and under the FairTax do not have to pay any tax when they take the money out under the FairTax. Also, the individual that paid income tax on regular income and on the interest earned (before the FairTax) and later pay the FairTax when the money is spent.
The worst case would be if Congress passes a law to take all the monies in retirement accounts of all kinds and use it for wealth redistribution before we can get the FairTax passed.