Actually, more likely that GT failed to meet quality or quantity that they promised, so Apple wasn't required to buy it. If they satisfied the quality or quantity's stated in the contract, then Apple chose to not buy it, there would almost certainly be a clause in the contract to say they could then sell to other people.
A supplier always have quality requirements in their contract. That way they're not paid for producing utter crap useless to the buyer.
As for Apple not allowing the Sapphire to be sold... If it is not good enough for Apple, will someone want to buy it? Maybe, but It depends on why Apple rejected it. Possibly good for something else than a screen.
As for exclusivity. Apple surely doesn't want GTAT to flood the market with Saphire essentially paid with their own money!!! Sapphire competitors can buy. Since they have a much lower volume, they could actually be satisfied with GTAT's outpout if it is merely a yield issue.