Isn't the idea of a bug bounty at this scale that the monetary reward (especially combined with the lowered legal risk, but also when considered in isolation) is higher from reporting it to the vendor than from selling it on the black market? I.E. presumably Apple has done their research and one million dollars is more than they believe you'd getting selling a zero day to somebody else.
I don't work in the security field nor am I a business number cruncher, but that was the gist I had of what these programs achieved.
Edit: see Despegar's reply, I should have RTFA! However worth pointing out that there would be some incentive for researchers to go to Apple instead of a third party, which might tip the scales in their favour.