The Reuters report has some details about why they limited it:
>Apple said it decided to limit the scope of the program at the advice of other companies that have previously launched bounty programs.
Those companies said that if they were to do it again, they would start by inviting a small list of researchers to join, then gradually open it up over time, according to Apple.
Security analyst Rich Mogull said that limiting participation would save Apple from dealing with a deluge of "low-value" bug reports.
"Fully open programs can definitely take a lot of resources to manage," he said.
http://www.reuters.com/article/us-cyber-blackhat-apple-idUSK...