On the contrary, you prohibit exactly "unlawful <sort of action>", e.g. "unlawful consumption of narcotics". That means that what exactly constitutes unlawfulne

On the contrary, you prohibit exactly "unlawful <sort of action>", e.g. "unlawful consumption of narcotics". That means that what exactly constitutes unlawfulness of such acts is a part of a different law, and usually not even a law, but a by-law or a regulation. For example, look at the German penal code, title 184f, "Ausübung der verbotenen Prostitution" — it's literally "Practicing illegal prostitution", and it's worded that way because there is also legal one.