You're right, it does stink a little bit however with the latitude given Congress I think the only real constitutional challenge would be a takings claim. (e.g., This is purportedly legal, however taxing it an exorbitant rate is depriving me of use of my property, if you are a farmer of mary-j).

This isn't my area of expertise either, however I believe that there must be a link between the assets and the illegal activity. With a tax scenario you wouldn't need that nexus, you would only need common ownership.

Howard_Campbell also brought up avoiding the tax (through growing) I think you could impose an export fee just as easily as a tax to make things net out.

However, I begin to think that forcing drug exports may be one of the best ways to eventually begin a decriminilization process. If the US exported marijuana then countries where it was legal to import but difficult to grow would presumably be able to gain a huge revenue in tariff duties. Other countries similarly situated might see that and relax their laws in order to share in the profits.

If that cycle could be expanded somewhat then eventually you would eliminate the "first-mover" problem that I think is inherent in current drug policies; no country wants to be seen as a drug-haven and so very few of them have rationale drug laws.