This seems to be an example of poorly supported prescriptivist grammar.
From The Cambridge Grammar of the English Language p192-194, a modern descriptivist grammar,
Under this heading we consider those uses of will where dispositions
or properties of the subject-referent are involved.
 i Jill won’t sign the form.
ii They have found someone [who will stand in for you while you’re
iii I will be back before six.
Example [i] implies unwillingness or refusal on Jill’s part; in[ii]
will might be glossed as “is prepared/willing to”; and in [iii] the auxiliary conveys the idea of intention.
 i I WILL solve this problem. [strongly stressed modal]
ii Will you lend me your pen? [closed interrogative]
iii I’ll wash if [you will dry]. [conditional protasis]
A strongly stressed will, especially with a 1st person subject, tends
to convey determination. A closed interrogative, especially with a
2nd person subject, characteristically questions willingness and
indirectly conveys a request (Ch. 10, §9.6.1). Futurity will rarely
occurs in a conditional protasis, as noted above, but volitional will
is quite unexceptionable, as in [iii], where your willingness is
clearly part of the proposition that is conditionally entertained.
Extension to inanimates
Volition implies a human or animate agent, but
something akin to a metaphorical extension of volitional will is found
with inanimates when it is a matter of satisfying
human wants,as in The lawnmower won’t start (someone is trying to start it)or >The books won’t fit on one shelf. These again appear freely in
conditionals: Give me a call if the engine won’t start.
The examples given in [38-39] (excepting the first person ones) can all be interpreted as expressing the intentions of other people.