FWIW I think this is intentional and a feature, not a bug. By spreading content to communities, you can delegate moderation responsibility much easier.

Content not posted to any community would need something akin to a site-wide moderator or an admin to moderate, and such a moderator wouldn't be as effective. They'd cover a wider array of very different content. Community moderators work better because they can define rules that are only confined to their comm and they know better how to moderate their own community and they also care more about their own community so are more motivated to keep it well-moderated in the fashion they want.