The basic bargain of the treaty is that those states commit to getting rid of their nuclear weapons eventually while the rest commit to staying non-nuclear.
Furthermore, the tax is considered to be disproportionate as funds could alternatively be raised by other means of budget attribution without affecting a basic freedom of the Treaty and, in any event, because the scope of the tax would be unrelated to the risks to be covered by the tax revenue raised.
It's not over yet -- Mr. Bush is said to be making a decision this week -- but the basic thrust is as follows: The U.S. would agree to delay withdrawing from the Treaty in return for Russia allowing the U.S. to proceed with anti-missile tests the Treaty now bans.