By "can't say so", I mean "can't say so and retain any power", which to a politician's ears is pretty much the same thing.

It's another example of anti-silly bias. Cannabis legalisation has roughly the same support as banning abortion, but anti-abortionists are treated as proper participants in a live debate, but people who oppose drug prohibition are still treated as silly stoners. No-one seriously tries to answer the arguments against these laws; they just fulminate about "irresponsibility" and "our children" and play the silly card.

If the very slow shift of demographics hits a tipping point on this issue, we could see some fairly rapid change.