The car manufacturer can approve towing or not for each model. Renault chose to allow towing on all the Clios, except the Sports models. And it made a towbar that fits them, all including the Sport. So you can buy a towbar, and you can get it fitted, but you can't legally tow with it. You could quite legally put a bike rack, but you can't tow with it.
Renault has chosen not to say why it didn't approve the Sports for towing. There may be a good technical reason, or it might just be they didn't think the sort of people who wanted a sporty model wanted to tow anything.
I have a towbar on my 172. I got it to tow light kart and box trailers. There are only two problems I've seen. The first is that first gear is fairly tall on Sports to limit wheelspin, and that can cause some difficulties in stop-start traffic uphill. You've got to torture the living daylights out of the clutch if you're stop-start-creeping forward slowly uphill. The second is that the anti-lock braking seems to get confused with a weight on the back. But its still better than I've experienced on light cars with no anti-lock and a trailer on the back.