Since a few have asked. The list comes from the individual instance (mastodon.social in this case). The admin can make changes to the list, a user can opt themselves out of it. It's not separated by any kind of “category”. Outside of the two company accounts we just show what the server returns.
My gut says that anything short of having a diversely populated timeline on launch is going to cause most users to run the app once, delete it and then go complain on Twitter that Mastodon is too hard.