I guess they have no tolerance for drug use in their country. Exporting it makes sense from an economic perspective. I believe US and EU(i.e. with GMO prosucts) is doing the same.
That is: no tolerance for foreigners smuggling drugs to China and contributing to problems of domestic drug addiction. China historically fought wars against the British Empire over this issue, and see it in a context of anti-colonialism.
Exports are one thing, addicts inside China are another. They still remember how Britain used opium addiction to control them. The Communist solution was to kill all the addicts after they took over.