Because Facebook presents itself to users as a way to share and express what matters to them. Not as a way to share and express what matters to them so long as it agrees with Facebook's censors.
I can't really seem to recall facebook making any promise about being a neutral party.
Additionally, the Facebook feed is not a linear display of content. The feed has been manipulated for a while now. So users have known for a long time that Facebook changes what they view according to Facebook's whims.