From the correspondence between an author and an (anonymous) referee:
Whether the prescribed usage had basis in common usage or not, it is largely ignored by American, Irish, and Scottish speakers of English, who are a majority of English-speaking people. The Pocket Fowler’s Modern English Usage, OUP, 2002, says of the rule for the use of shall and will: “it is unlikely that this rule has ever had any consistent basis of authority in actual usage, and many examples of [British] English in print disregard it”. The rule has even less force in American English, where shall has a much more restricted role, and the negative contraction shan’t does not occur. Indeed, in America, “I will” and “we will” are the usual forms, and anyone using “shall” in all but a few situations runs the risk of being thought haughty or pretentious.