It's assuming that those who can't afford to have children will be able to access birth control or temporary sterilization. That isn't the case, and the very people who propose this kind of thing are usually adamantly opposed to free birth control or state-sponsored abortion. People will not stop having sex. Sex is not just for procreation, it's for bonding, it's for the need to feel close and loved and warm, and it's a universal human need.
What this will do, however, is to allow the rich to steal with impunity, and force the poor to pay for their roads and all the infrastructure and the military. It's an obscene proposal that could only come from the mind of someone with no logic and no empathy. It will increase the national debt, too, because there is no way that the five percent from everyone would cover the needs of a civilized country; well, either that, or the taxes would have to go up, and the poor can't pay those taxes. What the hell do the proponents of a flat tax think that the average person is going to live on? Air?
Of course, the way that is proposed, if the birth rate of the poor dropped, the taxes would drop, too: The rich do not usually have more than two children, the Romneys being an exception. Two is the usual limit, perhaps three; one of the reasons the rich remain rich is that they control their fertility. It really has to do with passing the wealth on, and ensuring that their offspring are taken care of. They will not increase their birth rate so that they can pay more taxes! It is another way to shift wealth to the top, however.
I realize that wealth transfer is a dirty word in the U.S., but civilized societies that wish to remain civilized do not permit the kind of wealth disparities that are now apparent in North America.
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