IP is a huge governmental tax on the general public that accrues to a small number of wealthy people. (Any governmentally-enforced monopoly would be the same.) Taxing it would effectively reduce that somewhat, but it's easier to simply reduce IP in the first place. Make copyright 5 years instead of infinite, for example. That has the dual benefit of bringing a lot of content into the public domain and also being similar in effect to a tax on IP. (Think of it, if you like, as a 0% tax on IP for the first five years followed by a 100% tax on IP.)