I believe the "share" functionality is a new addition to iOS7. Previously (ios6), the only functionality available was "Open In" which allowed for invoking any other app that registered to handle the file type, the same as Blackberry's share functionality.
With iOS7, they added the dedicated share functionality, which was a quick way to share content. To get an app listed in this section, it needs to be curated by Apple. The "Open In" functionality is still available as an option for any app that hasn't been cleared to use the share functionality. Share just makes it quicker for a few of the most popular apps.