That may actually be a contractual/legal issue between Frontier and the console vendors and may not be moot regardless of the number of players involved.If console players don't have a MP pass, then surely that is on them, rather than restricting the game for the rest of us?
CMDRs without Horizons shouldn't have access to planets - can't see how Console players without MP access would have access to Powerplay.
In any case, in the OPs version there is still an aspect of PP they can do from solo so this tired old argument referring to a very small amount of players is surely moot?
The difference between your horizons example and Powerplay in MP is that horizons is a separate DLC and would be considered additional content outside the base game. Powerplay (making an assumption here, so correct me if i'm wrong) being a part of the base game, the console vendors may require Frontier to deliver the complete set of game features in single player mode.
I'm not saying this is the case, just that there may be more to "why cant they just..." type questions than meets the eye.