One of Nixon's aides flat out admitted, decades after the fact, that the war on drugs, especially marijuana, was instituted (or rather, ramped up) in order to marginalize blacks and hippies.
And I'm not even saying that your assessment is wrong, just incomplete. That belief (the politicians thinking that drugs are bad) is largely manufactured, and much of the blame can go to Henry J. Anslinger, yet it has not been sufficiently re-examined by governments in the light of new evidence[0]. Is that out of simple stubbornness or laziness? Or is it perhaps that it still serves the purpose of the state to keep these things illegal? Or is it simply that the state is loathe to give up any power once it has taken it, or even that governments move slowly and we just haven't hit the tipping point yet?
[0] The book to read is "Chasing the Scream: the first and last days of the war on drugs".
The trouble with these theories (I nearly said "conspiracy theories"), is the evidence doesn't fit the theory. If there's a secret cabal of "certain people" deciding this stuff, who are they? Where are they?
A more plausible explanation is to take politicians at their word. They really do thing "drugs" are "bad", that only bad people do drugs, and won't someone think of the children? And they've not really thought about the topic beyond that.
There are cabals, like the alcohol industry which infamously funded an anti-ecstasy campaign in the UK, but they are not hidden.