Here is the definition of "requirement" from Webster's: "that which is required; a thing demanded or obligatory."
Definition of "mandatory" from Webster's: "authoritatively ordered; obligatory, compulsory."
Same thing.
Dictionaries are not the end all nor are they absolute. Take it from Pinker as he sits on the board for the American Heritage Dictionary.
Steven Pinker (Guest Speaker)
I think dictionaries should inform the user what people's expectations are, and let them use their judgment. That is, flag a meaning or usage when it is generally considered uncouth or incorrect, or considered incorrect only by a few remaining pedants, and so on.
After all, a dictionary should provide information, not be a policeman. The "usage notes" in the American Heritage Dictionary are a good model -- for a careful writer and speaker, they provide information on how others are likely to react to a usage, and you can do what you want with that information.
And a video of Pinker stating that Dictionaries are not the end all or the absolute:
http://fora.tv/2008/09/12/Steven_Pinker_The_Stuff_of_Thought#chapter_09Also, Thomas Jefferson thought that dictionaries were quite annoying and that every citizen should create their own words and meanings as they see fit.