I'm not sure what you mean when you say that it's okay to mind-control a PC through magic.
Referring to kerfuffle that happened when Ross refused to let a PC control another PC's action, Ross recently said that it would be ok if they had an actual power. Everyone seemed to agree(though that may be because they are all tired of the argument)
The idea is that you want to avoid taking away the player's agency through purely narrative contrivance. You're trying to avoid saying, "Your character does THIS, because that's what I want to have happen for the plot."
That's why there's a difference between compelling a PC to do something through mundane persuasive pressure from an NPC, versus compelling a PC to do something through an established in-world mind-controlling power. The former is walking the border of narrative compulsion, whereas the latter is further away from that border.
Another big reason is that a lot of games have a very under-developed and unbalanced method of resolving basic persuasion, so that the target of it doesn't really have a fair way to defend itself. The game simply isn't designed for that to be a vector for the GM to attack the PCs with. For example, in Pathfinder, the only thing that helps against Diplomacy rolls is the target's base Charisma bonus. If I as the GM decided to build high diplomacy monsters to "diplomance" the PCs, they would be at my mercy. They probably couldn't come up with a reliable way to counter it even if they tried. Whereas there are lots of potential ways to defend against mind control spells.
Note, though, that there are some games in which it would be totally acceptable to compel PC actions through "words." For example, Burning Wheel has an elaborate system for verbal combat, called the Duel of Wits. In systems like that, persuasion is built to be a legitimate mechanical form of conflict. If you're playing a game like that, everybody can accept that there's a different social contract in play.
Hope that helps.
A bit. I was referring to hot PC on PC action though. I guess I understand in a system that isn't meant for it not having it, but it seems to me that just means you need to fix the system, not prevent the action.