On Hubzilla, there is a way to make the public stream available and only show posts from that server. But most Hubzilla admins never turn it on and probably don't know it exists.
As an administrator, the only time you would want to turn on the public stream is if you are a public hub and accept new signups. It makes it easier for administrators and moderators to moderate the public content on their own server since they can see all public posts in one place. If someone is posting illegal content or spam, a moderator can see it, and remove it (and perhaps the user too).
But private instances don't need this since everyone on the server is trusted.
@Comment105 Commercial interests and propaganda will come to the fediverse. The propaganda is already here if you know where to look.
The biggest strength of the fediverse is that administrators and users have control over what they consume. They are not force-fed information from third-parties without their consent.
So, companies and politicians are going to show up on the fediverse. But you don't have to follow them.