And just to clarify it a little more.
When creating a Plex channel, you can only request the raw data provided by the online source (the same data that online source uses to populate its web pages). You then use the Plex plugin framework/API to create the menus, listings, metadata, and media from that raw data you have requested and pulled from the online source.
Therefore to access any data specific to a user or login, you would have to determine a method for sending user authentication data as part of that request for the raw data. And the majority of online sources do not openly provide access to the user authentication methods used to access their raw data.
Other media player programs may provide methods to bypass the need to send user authentication data by replicating a browser interface, using Flash, or Webkit. But, the Plex plugins framework/API does not provide any means of replicating a browser interface. And Plex does not use or support Flash or Webkit.
The specific methods that must be used when accessing data and creating Plex channel plugins are why Plex channel plugins appear much cleaner, more easily navigatable, and usually require less resources than the online media offered by other media programs. But it also makes the methods necessary to access the data to populate these channel plugins much more difficult.