Re taxes etc, I always thought "the thin end of the wedge" summed it up perfectly.
Having taken the trouble to look at Wikipedia, I see it says "positive reinforcement and/or suggestion" works as well as direct action like legislation.
So I think what its getting at is maybe, for example, having happy heart trails, encouraging the explosion in jogging/running, having shows like Operation Transformation might do more good than, say, a fat tax.
The behavioural effect of taxes - e.g. plastic bag levy - is probably being outweighed as a motive by the need for cash. So if we had a fat tax they'd probably want us to all be porkers and collect loads of cash, as opposed to guiding us away from unhealthy food.