ZenithSlinkyDinky He's talking about why there is a release section for the various MyBB Versions if it's free (1.6.x and 1.8.x), not the plugins and themes.
And I have a feeling it's mainly due to the fact someone might not know when a new version of MyBB has been released, or where to download it. It also generates SEO for the community, which in turns allows for more members to join. It doesn't hurt anything, so I just wouldn't think too much about it.