Help & troubleshooting for channels on your Roku device, including adding/removing channels, logging in to, authenticating, or activating a channel, channel-specific playback issues, assistance contacting channel publishers to report issues, and adjusting channel-specific settings.
You can get a provider or see if they offer other options. (For example, you can watch CBS with a provider login or you can sign up for CBS All Access without one. Such options vary by company.)
As for why – most of these companies are used to dealing with (ie: getting paid via) satellite and cable companies. So, if you're already paying, they let you in. But some also have a direct to consumer option as well. There is also completely free stuff on Roku.