>I can't really imagine it's for any other reason than to have a firmer control on the population, or an excuse to imprison people or criminalize them in a convenient way.

I assume it's mostly to ensure your culture has a suitable fighting force. If half of the warriors are drunk or stoned, you'll lose whatever battle happens to come up.

It's only in the last hundred years or so that most nations have made most drugs illegal (there have been many exceptions of course but I'm talking about that mass banning across almost all nations that we see today).

Hell, it wasn't that long ago that the British went to war with China for the right to keep selling their citizens opium.