Why will some people accept a rule in the book but not the GM's fiat?
That's a good question, and I'll do my best to address it. Many people rely on the rules as a form of support, be it player or GM.
As a player, it lets me know what I can reliably do. If I know that attacking requires me to roll a base attack + Strength roll, and I need to hit their armor class, then I can plan around that. I can attempt to attain a higher Strength, or a higher base attack, or a masterwork weapon (in 3.x), or feats that boost my roll. It lets me know how I can craft my character to play out the way I want it to.
For example, if I had in mind a character who was this huge brute that hit people with a giant axe, I probably envision a high Strength character, possibly with a high Constitution. Now, if I found out (through rule or house rule) that Dexterity is the attribute that covers whether or not I can hit someone, than my concept changes to include a high Dexterity, so I can make use of my giant axe.
These written rules give players a strong starting point when determining the mechanical manifestation of the concept of their character. If their mechanical character does not match their conceptual character very much, than they often find themselves distanced from the character, losing that special connection, immersion, or whatever it is that makes that character special and spark in their mind.
For a GM, having the rules to draw on is incredibly comforting when you are learning. After you have played with the rules for a while, you can see the strengths and weaknesses of the system. Lastly, having rules in place allows you to cite them when a player disagrees, even if they think it is a bad rule (you have even asked EN World in the recent past how to justify a mechanical rule, rather than change it to something perhaps more realistic, here:
http://www.enworld.org/forum/d-d-legacy-discussion/304196-flat-footed.html).
Of course, it is true that the more you know about something, the more you can break the rules you were taught at the beginning. So, with more experience, it's much easier to break these rules and have an enjoyable experience, but I think that's a fairly universal rule.
As a player, the rules are their for guidance, and as a source of reliable material. You cannot reliably use GM fiat, for even an amazing GM must come up with the mechanical roll for you to use on the fly. You cannot know what the rolling mechanic will be any more than he does. Take, for example, the maneuver to dodge and have two enemies swing at one another. If I wanted to do that, and the maneuver had no mechanics, than I could have no way of knowing that you'd rule it a certain way, for even you have not decided what the ruling is yet (as the situation is only now coming up). However, as it is a maneuver, I know I can reliably use that mechanic, and I can temper a character concept by careful use of the reliable rules.
As a GM, they are there as a source of incredible guidance, giving you example after example of how the system envisions Dexterity being used, attack rolls being used, Constitution checks being used. The rules help you see the spirit of the game. Mutants and Masterminds use Hero Points to allow your character to do things beyond their normal control. There's a knockback mechanic for when you get pretty hurt by an attack. These help indicate the style of game the rules were created for.
When a GM begins to use his granted power of Rule and Overrule, I do not immediately buck against it. I'll accept it. If it creates an inconsistent or displeasing game experience, I'll voice my concern, and I'll drop the game if it continues. I will not try to take away that right from the GM, as he has the right to run his game the way he wishes. As a player, I think I should look for someone a little more in line with what I enjoy. Since, really, it's all about enjoyment.
And that's the crux of the matter, really. It's about mutual enjoyment, and it's about a consistent gaming world where reliable mechanics are incredibly useful. For all of my problems with 4e, I really, really like the idea of a unified mechanic for on the fly maneuvers (I doubt I'd like the implementation, but that was true of 3.x, 2e, etc. mechanics). Having a rule like that exist allows players to reliably use new abilities in a predictable method, and thus does not infringe on their enjoyment when something seems inconsistent. It prevents situations where an action was Dexterity once (because of the ability to react quickly) and Intelligence the next time (because of the ability to think quickly).
That's really it. I break or bend rules all the time, and I attempt to do so in a consistent way. I have the added benefit of playing a game of my own creation, so I can just fix previous rules as these new decisions are made. Regardless of that, I can say in all sincerity that I am an amazing GM, even though I embrace the rules. Honestly, my players love me GMing (I'm stuck never getting to play). I'd be a horrible GM to other people.
It's a playstyle difference. You wanted to know why people preferred rules over GM fiat. That's about it, I think. I hope it at least illuminates the issue, even if you don't agree with the playstyle. And, the real beauty of the thing, is that everyone gets to play however they want. And that's pretty amazing.
