Zone is new now though, a lot of people will be getting iTunes cards for christmas so I doubt Stefan will put it up until after the surge of people buy Zone+. He would lose so much money.
Since iClassic and iEra both have microtransactions, he can spread the popularity of the game by freeappaday promoting it, at the cost of people getting the plus version for free, who likely wouldn't pay for it in the first place. He also gets new players to find the game, who may spend money in the future.
Basically, iZone is too new to make the paid version free