Allowing someone to push branches to the "central" repository versus requiring them to have forks of it is functionally the same thing, assuming you set up branch protection for master.
Prior to 2 minutes ago, I had no idea you could lock down a branch in github. We have folks working on our codebase, and the team wants to enforce code-reviews for anything that goes into our master branch. To enforce this, only a few people have push access.
This is similar to the open-source maintainer model.