I don't see why we should have such a tax, a special tax is put in place when the consumers should feel guilty to consume the said product. It allows states to find easily money, the health being mainly an excuse. I don't see why I should find myself guilty for using kava, kava has no health or social drawbacks similar to , e g. alcohol, that could justify any shame on my side and therefore a specific taxation.
However, taxing kava could increase strongly the acceptance of kava by the reluctant governments (as in the EU), greed being by definition rooted in them.