I found this a very pertinent article by Polly Toynbee:

Bad politicians are slave to public opinion. Good ones try to change it

Her mention of the Alan Johnson & David Nutt clash is just what I said to friends when discussing it:

‘ Johnson could have told a truth about both science and politics: “This drug research is accurate, but people fear softening the law sends the wrong message. I intend to talk about it openly. Politics is about listening and talking to people. When the facts are more widely understood, we will review drug classifications.” ‘

Polly gives a few other examples of scientific evidence being ignored in the long term.