Ah, the diplomacy double standard. PCs expect their courtier to be able to change people's mind with a single roll, but consider NPCs doing the same to them mind-control.
1) Past experience for many players is that some GMs can be very heavy-handed or even abusive towards the player or player enjoyment with the use of social rolls (independent of game or system). Skeevy example that too many of us have seen -- "Your character sleeps with him because he beat you on the contested seduction roll... hurhurhurhurhur."
2) There's a thin line between "role playing" and "roll playing" when it comes to driving actions or decisions or "consequences" through simple social rolls, regardless of the directionality between PC and NPC.
3) An NPC is just that, a non-player character, and isn't the "dramatis persona" (to coin a twisted term) of a single actual real person.