Well, that's the nature of the game, any game with replayable levels that then had a level as demo would have that effect
Yeah, but think of the game as a "platform" with a number of different locations.
Each location grants you access not only to the story mission, but also unlocks, challenges, time-limited events, and you can create missions and access all user created content based on that location.
So, with this, they are essentially giving you the first location, without limitations. Unlike what we usually get with a regular demo, this allows you to access all content based on this location, and your overall progress and unlocks are saved.
If you get the rest of the game (or if you buy the previous game, or the legacy DLC, either granting you access to all locations from the previous season), you get the rest of the locations.
But, you can simply continue to grow your character just by doing content on the location you have access to, and even enjoying events (for example, on March 8 users with access to New Zealand will be able to play the upcoming Elusive Target).