Because a game that lets you beat up BOTH is just super awesome.
The big thing is, it's fun! Getting that subjective thing out of the way, it's fun managing the relationships with the natives, whether its in an exploitative (i.e. Spanish haha) way, or getting them to help you and help your settlements grow.
Playing the game of cat and mouse, and getting all the advantages the natives had against you, while fighting the national army after declaring independence is also fun. Scoring big and finding that mountain range with lots of silver, only to have to decide between the 7% increase in tax or having a "Silver Party" is one of those frustrating "**** you" moments, but in a good way.
What I meant was, post 3 and 4 indicatate that the unpleasant subject matter drives them away from that expansion. Why is that any more unpleasant that anything else? Was just curious.