Why legal checks before release?

Committing code to a version control system is a form of distribution, since it will be available to anyone. Distribution, in any form, is the usual trigger for copyright-based licenses, due to the nature of copyright in the law.

That's why we want the legal parts of the packaging job to be done before the package is approved at Savannah—notably including valid copyright notices and inclusion of the full text of your package license (ProjectApproval, HowToGetYourProjectApprovedQuickly).

In short, "release" in the legal sense is not only packaging a tarball.