It is important to remember that any American or Frenchman, who decided to exercise their free will and legally sever their tax liability would be liable topay all applicable taxes (income, capital gains, gift, wealth) right up until the moment that they left.
Indeed they are available to "taxpayers" who don't paytaxes, which is to say that in some cases they can result in a negative tax liability--an actual subsidy, as that word is commonly understood.