If a GM includes oppressive cultures or governments in their setting—especially if they oppress groups that the PCs(or players) belong to—and lets the PCs get to a level of power where the PCs are capable of fighting these governments or cultures and the GM doesn't have plans to either let the PCS do that or explain why they can't beyond just "no, you can't, cna't be done," then the GM has failed utterly in their job.

If my Chaotic Good Barbarian gets to level 20 and can't depose the king of so-called civilization who is pressing and hurting innocent people for reasons my Barbarian finds stupid, not just not having the opportunity but being physically prevented from doing so, even if her level 20 Sorcerer, Druid, and Brd friends agree with her, then the GM has made a mistake.

And that goes double if the oppressive King is intended to be seen as a "good guy."