To be fair, (at least in Germany) it was taxed even before it was legalized. To make tax collection easier they even created a simplified procedure (Düsseldorfer Verfahren) which is still in partial use today.

Just because you do something illegal doesn't mean you shouldn't pay taxes. I think there is something similar in the US, where you can "take the fifth" on a tax return. Kansas even taxes illegal drugs and requires dealers to purchase and affix "drug stamps" to them.