The primary workflow promoted by github seems to be to fork (using github's sense of the word) and use pull requests to contribute. This workflow, I imagine gets you to spend more time on the site, as opposed to using github simply as an ssh endpoint. This workflow also seems to be the biggest complaint about github by hardcore git users.

The change then looks to be simply an effort to highlight the workflow they want you to use.

I must however note that any effort to shout conspiracy should be tempered by the fact that github makes its money by selling private repos, so the above change, while reinforcing their suggested workflow, does not necessarily benefit them directly.