It was a rude gesture to begin with, and unnecessarily pedantic even if it had been correct. Worse, it was not correct. Pedants should take care to be correct, as other pedants lurk. So I dashed off a quick explanation of the interchangeability of shall and will in this instance.
I thought my audience here is more likely to be interested in the current state of the shall v. will argument in English usage than most other folks would be.
The short answer is that there is no difference. If THS prefers “shall” for reasons of its own, there’s nothing wrong with it. However, I wonder at the efficiency of changing wording because of such idiosyncratic preference.I know you are all grateful to know this.
The longer answer: There is a century’s worth of controversy about shall vs. will, and authorities can be found to support several points of view. All of them are quite sure they are correct, which is the usual state of affairs in questions of usage.
In everyday usage, the difference is one of connotation only. Shall seems more emphatic, and people use it to mark an emphasis, to make a command stronger. This may also be used in a mildly ironic way: Shall seems overformal and haughty to some, and its use conveys condescension or pedantry, usually in self-mockery. I use it in irony all the time. Younger people do not regard shall as stronger, however. The word will seems more emphatic to many of them – not my children, of course, but that’s the way the language is changing. I think it comes from less exposure to the King James Bible, even for religious children, which renders Early Modern English, such as Shakespeare, less understandable to them. The shall command is no longer in our bones.
There was an older standard of correct usage which dictated that shall be used for first-person futurity, reserving will for the second and third person. In the reverse, will was used for first-person obligation, shall for second and third person. The problem with this is that the south of England had the reverse rule than America, Ireland, Scotland, Canada, and Australia, but such was the prestige of Oxford and Cambridge that many speakers of English abroad imitated their usage. Your Strunk & White will still insist on this, but both the NYTimes and Chicago Manual of Style do not.
A residual shall/will distinction persists in law and regulatory writing, but several problems arise here as well. American and British legal use differ slightly, and shall is now officially discouraged in legal writing, except in America. Oh, and discouraged or not, there are many Canadians, Brits, etc. who refuse to change because they’re right, dammit, and they aren’t going to drop their standards for anyone. American lawyers are more likely to preserve the distinction that shall is used for obligation, not mere futurity, and is the stronger and preferred term. This is called the American Rule (its counterpart is the ABC Rule, standing for Australian-British-Canadian). Even in America this is becoming obsolete, however. In rental contracts, the use of “the tenant will…” or “the landlord will…” are regarded as unambiguous obligation.
In regulatory writing, particularly in contracts involving scientific or technical requirements, the distinction is preserved most strongly. Many technical writers and translators insist on the use of shall to indicate the strongest possible obligation, exceeding not only will, but even must.
Further note: etymologically, shall is related to “should” and will is related to “would.” They were used as modal verbs to show future tense in an English language that doesn’t really have a decent future tense. The distinction is remote enough that it should be disregarded in discerning current meaning.