Creative Assembly has addressed this publicly on Twitter. Strategy games take a long time to make and cost a lot of money; these particular ones even more so. Compared to other genres they command a very limited, niche market.

In order to reach as large an audience as possible they target a TEEN Rating (or your Country's equivalent). To keep that TEEN Rating, they cannot have BLOOD & GORE in the base game. If they offer BLOOD & GORE as a FREE DLC, the base game will be FORCED to an MATURE Rating as a result of Blood & Gore being readily available to a general audience. They are forced to charge for the Blood & Gore DLC to keep the TEEN Rating.

They charge the lowest amount they are allowed. They only make you buy it ONCE to unlock it in all 3 games. And what do you supposed fans of the genre do in response? 61% of you put on your mongoloid hats, apparently. I'm sure it'll be fine if they follow your business advice, right? Surely you are all successful business people and are well versed in the actual limitations that are applied to your marketing and sales options when your game goes from a TEEN to a MATURE rating globally, right?

--- Update ---
2 years later I'm enabling comments to possibly divert the apparently lazy, enraged, uninformed masses from commenting on my profile to ask me who told me CA has to charge for the DLC and that I'm lying to people with my review. I should have expected this since CA has been addressing this issue with the public for a decade.

The Total War series, as a whole, does not have Blood & Gore outside of optional paid DLC (Shogun 2 to present,) and in a very limited form in Medieval 2 (which was made by a different studio, the short lived CA Australia.) It's a separate DLC because of ratings.

Check the ESRB FAQ, if you include FREE DLC with your game the base rating must change accordingly. PEGI (Europe equivalent) and the Australia Classification Board have similar systems. PEGI is slightly more relaxed than ESRB. Australia is usually more to much more strict. Obviously all subject to change.

"When you're dead, you don't know you're dead. All of the pain is felt by others. The same thing happens when you're ignorant."