The reason that Creative Assembly is allowing this freedom is that the developer doesn't want to split their player base into those who own the expansion and those who don't.
“One of the things that we made sure of with Fall of the Samurai is that we didn’t split the online community. Players without Fall of the Samurai can play against people with the expansion. That’s really important to us. We want to make sure that people who buy the new game can play against the guys playing Shogun 2," stressed Creative Assembly lead designer James Russell.
Russell added, “Total War – particularly the campaign side – is a very unique, thoughtful, deep experience that we want to maintain. We definitely want to keep Total War as an immersive experience even though we want to create multiplayer battles that you can get in and out of quickly, and have fun if you’ve only got half and hour.”
Total War: Shogun 2 - Fall of the Samurai is due to be released on PC on the 23rd March. The base game, Total War: Shogun 2, earned a 9.10 score in its Strategy Informer review.