Of course, some of these guidelines are a lot more rule-like than others, and can very easily be mistaken for absolutes. Take grammar, for instance: there are constructions that are right and constructions that are wrong. Is there really any room for manoeuvre there?
Well, yes and no. I'd certainly encourage any writer to learn how grammar works, how to punctuate correctly and the actual meanings of words - many are hideously abused - but there are times when the rules are flexible. Flexible, but not infinitely elastic, and only if you know precisely when and why you're breaking them.
Sometimes it can be for effect. The last sentence in the previous paragraph is ungrammatical, because it's a fragment, but a fragment can sometimes be extremely effective, as can starting a sentence with and or but. The secret is to do it at exactly the right time and to know why you're doing it. The two secrets are to do it at exactly the right time and to know why you're doing it, and to use it very sparingly. The three secrets...
The other main reason for using incorrect grammar is register, or the level of language-use you want to convey. The most common distinction is between narrative and dialogue, but there's a great deal of variation within these. Narrative might be the formal relation of an old legend or a chatty first-person account. Dialogue might be spoken by a pedantic academic or by a yokel, or anywhere between. In some cases, correct grammar isn't appropriate.
I'll take a simple example - the word like. If you're following grammatical rules, it's correct to say He ran like a demon but wrong to say He ran like a demon was after him (here, like should be replace with, for instance, as if). However, many people use the incorrect form, and in some circumstances it might give a feeling of the character or the narrative atmosphere to use it. If you don't know it's wrong, though, you're likely to use it where it's inappropriate.
If even the rules of grammar are flexible, what about all the other "rules"? Don't use passive voice; cut out adverbs; don't use words ending in -ing; don't use past continuous - and so ad infinitum.
Most of these have some reason behind them, but they're ridiculously dogmatic. It's rare to find a construction or part of speech that isn't effective in its right place (I'm tempted to say it never happens, but I wouldn't be that dogmatic) but some are easier to misuse than others. That's all.
Take the much-maligned passive voice. Now, for a start, many writers (and even some editors) don't know what the passive voice is. It isn't "writing weakly" or "distancing" - it's the form of the verb where the subject is the person/thing acted on, as opposed to active voice, where the subject is the actor, and middle/reflexive voice, where the subject acts on itself. An active sentence would be John hit Fred. The passive form would be Fred was hit by John. Middle would be John hit himself (idiot).
Neither active nor passive is right or wrong: they perform different functions. In the example given, it depends largely on whether the sentence is about John or Fred - whichever we want the reader to focus on should normally be the subject.
Beyond that, though, passive can give an impression. The anti-passive sentiment originated in business writing, where it's important to be strong and positive all the time, but this doesn't necessarily apply to creative writing. Imagine a character who appears to be a strong leader or general - but you have your doubts as you read. Something about him doesn't seem quite as strong as he makes out, and it turns out in the end that this impression is correct. Where did it come from? Simply that, throughout the story, a lot of statements about him, or things he says, are passive constructions.
This kind of thing applies to all the "rules", so where did they come from? I suspect a lot of them may have originated in mistaking specific advice for general advice. An author might get a critique from a Respected Source (a top editor, or a published author) who criticises a battle scene because "You use a lot of passive constructions, and this slows down the action and makes it seem remote."
Now, this is perfectly good advice: there are valid places to use passive, but a battle scene isn't one of them. However, the author doesn't quite get that and spreads it all over the internet that "Respected Source says using passive slows down your writing and make it seem remote." This isn't what RS has said at all, but it's plausible enough to catch on, especially if Acknowledged Expert and Impeccable Authority can be similarly misquoted.
Thus a "rule" is born. It's not entirely wrong, but the correct guideline should be "Keep an eye on your use of passive voice. It has its place, but avoid it in sections where you want the action to be fast paced and vivid. If in doubt, it's probably best to change it to active."
The same is true for all the other "rules" that are flung around: guidelines, not rules. In reality, I've only ever found two genuine rules for writing. One is If it works, do it; if it doesn't work, don't do it. The other is Never believe a rule that contains the words always or never. Including this one.