Its not about running costs, its about environmental impact (although a tt cabrio was 5k more than a same engined coupe when I looked for examples). The point is that 4x4s are blamed not just because they have nobbly tyres and big engines - but because they are used by people who balatantly don't need the nobbliness of the tyres or the strength of the engines (unless the kid they drive to school is really really fat, in which case - my condolences).
This is why the cabrio situation is quite similar - you have significant resources being spent to satisfy two desires - posing and the feel of sun/air on your skin. Neither are that essential in the great scheme of things.
I am not just trying to say all cabrios are bad - I would love an MX5 myself, just giving a different perspective.
P.S. I can't see how freedom of speach would have anything to do with what car you are allowed to buy though
