The gist of Apple’s bounty programme is that they want to compensate people for their work, not pay them the black-market value of the exploits. Because usable exploits are becoming increasingly harder to find, only experienced hackers or programmers can find them, people who could easily use their talents elsewhere. The bounty programme is Apple’s acknowledgement that hackers have to spend real time to find these, hence why the rewards scale according to technical difficulty, as an encouragement to look for them.
If I find an exploit matching this criteria, why would I report it to Apple if I can make so much more selling it to these people?
What's the incentive?
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One reason I can think of is that if Apple gets into a bidding war, this might drive the price up and draw even more hackers to try and find bugs with iOS. Which would in turn expose even more flaws with the operating system, opening it up to even more vectors of attacks.
Second is worth. Just because some company out there is willing to pay 1.5 million for an iOS exploit doesn't necessarily mean that Apple thinks said exploit is worth that much to them.
Than we will have the San Bernardino type of court battle all over again.