And, right here, this is what people are talking about with DM force.
The mechanics are pretty clear. If the player rolls high enough, the character's attitude is changed and will outright become helpful.
But, force DM's will simply interpret "helpful" in such a way that it pretty much leaves the NPC's actions unchanged and unchangeable. The DM will, while possibly adhering to the letter of the rules, simply adjudicate in such a way to negate any possibility of success. All the while justifying his or her actions by appealing to some notion of "this is the way the world is" and "this is what a DM is supposed to do".
I am unclear. If a DM abides by the letter of the rules in such a way as to arbitrate against the players, then that's DM force? So anytime the rules are against the players, the rules should be discarded, but anytime the rules are against the wishes of the DM, the DM must give in? That seems to be what you are saying. I am not sure though, that this is what you actually mean...
Since we are on Diplomacy, let me agree the rules are pretty clear. They say:
"You can change the initial attitude of nonplayer characters with a successful check. The DC of this check depends on the creature's starting attitude toward you, adjusted by its charisma modifier. If you succeed, the character's attitude toward you is improved by one step. For every 5 by which your check result exceeds the DC, the character's attitude toward you increases by one additional step. A creature's attitude cannot be shifted more than two steps up in this way, although the GM can override this rule in some situations." - Pathfinder Core, Diplomacy, pg 93.
Let us notice, as already pointed out above, that the starting attitude is a matter of DM arbitration. Moreover, if a creature is hostile, the use of Diplomacy never makes a creature friendly,
unless the GM specifically chooses to override the rule. The rules as written more or less states that Diplomacy, as a skill, is subject to DM arbitration from beginning to end.
A little later, next page, we read,
"Any attitude shift caused through Diplomacy generally lasts for 1d4 hours but can last much longer or shorter depending upon the situation (GM Discretion)." Note, that I did not add GM Discretion, it is in the rules. And it is repeated in the next paragraph,
"Some requests automatically fail if the request goes against the creature's values or its nature, subject to GM discretion." Then the next paragraph,
"The GM might rule that some topics are simply unknown to common folk."
The rules could not be more clear about Diplomacy: The DM is in charge of arbitrating its use.
Now, when players give the DM this sort of control, there is the issue of trust. The players must trust the DM. Its a big job (relatively speaking, it is, after all just a game), he needs to take that responsibility seriously. When one of my boys was practicing DMing so he could run games at a Con, one of the things I got onto him about was making arbitrary decisions just because he could and he thought the players were getting by too easy. He was abusing the power and needed to cut it out or it would not have been fun for the players. The authority to do a thing does not mean you should always do a thing, and, as Gygax noted in the AD&D DMG, as Cadence shared a few pages back,
"Know the game systems, and you will know how and when to take upon yourself the ultimate power. To become the final arbiter, rather than the interpreter of the rules, can be a difficult and demanding task, and it cannot be undertaken lightly , for your players expect to play this game, not one made up on the spot."