[sfn42]

Well, it is. If opium is more valuable than food crops, farmers should grow opium. Then they can buy the equivalent amount of food and still have money to spare.

That's how the economy works. It's called opportunity cost. You should always create the most valuable thing you can, assuming of course that you can sell all of it.

[rawTruthHurts]

Absolutely. Self-dependence is overrated anyway. If they find no buyer they'll be able make some nice infusions.

[sfn42]

Self-dependence is more of a state-level concern. If the state values self-dependence they can subsidize and otherwise incentivise types of agriculture they prefer.

Outright prohibiting a crop obviously also serves that goal but it also weakens the states economy and could cause tensions if farmers lose profits.

And there are uses for opium besides street drugs. Opiates are used a lot in medicine.