Grammarians at the Gates

Who decides whether it's acceptable to end a sentence with a preposition or to use the word "infer" as a synonym for "imply"? Who decides whether the phrase "free gift" is redundant and therefore incorrect, and whether it's proper to speak of a "mutual friend" since "mutual" refers to a relationship between two, not three? Most literate people still want these questions decided for them by some authority, whether H.W. Fowler, the usage notes in the American Heritage Dictionary or the guy in the next cubicle who knows a lot about grammar. This urge for clarity remains despite the best efforts of academic linguists and other "descriptivist" grammarians who dismiss the notion of grammatical "correctness" and insist that "rules" are wholly determined by usage.