Dictionaries aren't authorities on language.
WTF?
Of course dictionaries themselves aren't authorities on language, they are after all books written by people; but the reputable dictionaries are written by people who ARE authorities on language, including structure, usage, meaning, etc. What they do is look to the use of language--ones like OED and MW look at how words are used in the language in general, and identifies the most commonly used senses. Different versions of a given dictionary are more comprehensive than others (abridged or unabridged, for example--the unabridged OED is huge; the MW Collegiate edition is by comparison shorter and incomplete, lacking many of the less common usages and not making sufficient distinction. Online dictionaries, such a dictionary.com, are so basic as to be almost useless except as an quick reference), and the quality varies depending on the organization that has undertaken to make the dictionary. Other dictionaries are dedicated to the use of language in more specific areas, geographically or topically or professionally, for the most part.
One of the biggest drawbacks of dictionaries is that language (especially English) is huge, complex (it isn't just one language after all, it's parts of seven or eight other languages forced together at gunpoint), and is constantly developing new words and bringing new words in from other languages) and always changing in other ways, such a grammar, style and pronunciation, but it takes experts years to identify and develop adequate examples of usage to put into the dictionary because by comparison to the task, there are very few experts devoted to keeping dictionaries current.
Inevitably, there are going to be usages listed in the dictionaries that are not correct to one person or group's usage, and others will assume that the dictionary is the final authority. That's not the dictionary's fault, nor the authors' fault: it's the user's fault. And maybe the education system's fault for holding up dictionaries and encyclopedias and the like as being Authorities From On High.
That dates back to the first dictionaries and rules of the "Proper" use of the English language, which were developed in the early 1600s so that 1) the growing British Empire could be more effectively administered, and 2) everyone would be able to tell who was EDUCATED (and therefore of the upper and middle classes--and you could tell which because the education and socialization was different for those two groups) and who was not, who was of the working class...or foreign, from the colonies, and so on.