Let me address that in reverse order. Very little prevents the states from applying different rates. Some may do it through the sales tax or may use other UPC-based taxes like Florida's flirtation with the Advanced Disposal Fee. Look at how states, counties, and even cities and towns add tiny earmarked fees to things like property tax (a 1/2 mill surcharge for some pet project) or to telecommunication charges (Internet access fees, local surcharges, etc.) The model exists and the only thing keeping it in check are the voters with their last lines of defense like referenda requirements. My county requires a vote to activate the local 1% adder to the sales tax, for example, and we have turned it back many times. This year, the Florida legislature tried to take that option away from the voters because it has proved to be an effective check on county spending. It is a very thin line of defense and under attack all the time.
What holds back the Federal government is simply the political will to implement a federal sales tax. Note well that there is no federal initiative petition process or way to float a referendum. I am actually surprised that the Federal Sales Tax proponents haven't tried to implement a modest tax to get the process started. It is likely that it would require a Constitutional Amendment in the same manner that the 16th Amendment was required to get around Article I, Section 9. Perhaps it is viewed as too much effort for a small tax rate coupled with the public's automatic aversion to tax increases, albeit a small margin.
BTW, can't you just picture Hillary floating the 1/10th or even 1/100th mill surcharge for breast cancer research then someone else doing so for AIDs research, then Embryonic Stem Cell research. Tiny earmarks for "noble causes" that will generate vast sums of money to keep special interest groups happy campaign contributors. Imagine trying to roll back one of those taxes? It is much simpler to implement such a revenue scheme under a sales tax than it is under an income tax. Today, they do it via spending bill and it is hard enough to zero out funding for pet projects. But a sales tax approach makes it a much easier way to make earmarks permanent. Remember, the passage of the "fair tax" will not change the nature of Congress, it will simply give them a new toolbox and when I look at the tools inside, I get a good idea of how they can be misused.
All you needed to say was: "Nothing and Nothing."
"It is much simpler to implement such a revenue scheme under a sales tax than it is under an income tax."
"It is a signal advantage of taxes on articles of consumption, that they contain in their own nature a security against excess. They prescribe their own limit; which cannot be exceeded without defeating the end proposed, that is, an extension of the revenue. When applied to this object, the saying is as just as it is witty, that, 'in political arithmetic, two and two do not always make four.' If duties are too high, they lessen the consumption; the collection is eluded; and the product to the treasury is not so great as when they are confined within proper and moderate bounds. This forms a complete barrier against any material oppression of the citizens by taxes of this class, and is itself a natural limitation of the power of imposing them."
Alexander Hamilton in Federalist #21
"Remember, the passage of the 'fair tax' will not change the nature of Congress, it will simply give them a new toolbox and when I look at the tools inside, I get a good idea of how they can be misused."
Actually, the FairTax will greatly reduce the congress's "toolbox". The area of greatest complexity in the current system is in the area of deductions. That opportunity for mischief is totally absent in the FairTax. Will congress attempt to compromise the simplicity of the FairTax tax base? Undoubtedly, some in congress will. However, with the difference in public perception that the FairTax will create, there will be a much greater recognition of the price that we all pay for the special interest provisions in the old system.
Bottom line is that the FairTax will be much more simple and straightforward than the current system, even if we do a poor job of maintaining "eternal vigilance". If we do a really good job in that area, we can keep the system relatively "pure", which would mean an enormous improvement in that regard.