Turning that on by default would be against Facebook's Platform Policies.
From https://developers.facebook.com/policy/ :
IV. Application Integration Points
3. If a user grants you a publishing permission, you must still obtain consent from the user before taking any action on the user's behalf, such as publishing content or creating an event.
App does not automatically post Stream stories on a user’s behalf and instead obtains consent by providing users with an option to click a button or check a box that clearly explains their content will be shared.
They go into it in other places too, but the basic gist of it is that the user must take affirmative action of some kind to show that they want to post the story to their stream, you can't opt them in by default.