Well, if you start talking about Reagan in the 80s, there's significant evidence. Not as a way to channel unemployed, but to have extra governmental funding for covert operations.
If individuals mostly make decisions based on a number of internal currents, conscious and unconscious, the same must apply to collectives.
Is there any evidence of such a "rise the lowest forbidden fruits won't impact the crop"?
Without data, one could just as well argue that it favors criminal paths by smoothing the road.
Think of it this way: if you rule death penalty for illegal music download, you further banalize evil by broadcasting the message that the worst social behaviors is ethically equivalent, as far as law pretends to pursue some ethical aims, as something as harmless.
Nota bene: I don't imply that death penalty is acceptable for any kind of judicial sentence. Human errors and possibility of political abuses should be sufficient points to definitely end such a practice out of any legislation that pretends to respect its citizen dignity.
I would like to make a point that the effect doesn't have to benefit everyone equally. Historically powerful factions of society have pushed things to the detriment of the majority and the furtherance of a tiny minority.
That the elites of the world are happy to have drugs being illegal, as it is a way to channel unemployed, violent youth who want to make a quick buck.
I mean, think of it this way: If you are a poor, violent and traumatized youth, and drugs were legal and you wanted to make a quick buck, you'd turn to robbery or worse, kidnapping/murder. As long as they prohibit something that people want, that cohort will always have a way to make a quick buck without turning to robbery/kidnapping/murder.