I am on my MOC's nomination board and I usually ask that question.
Two things,
In past 6 or 7 years, I only agreed with one applicant's rational wahy he was only applying to one academy.
Most cases, we didn't penalize any applicants for wanting to only attend one SA. We agreed that kids will be kids. At the same time, if an applicant went down a wrong path, it did hurt the applicant. A short story, an applicant that only wanted attend USNA to fly F-18. Nothing wrong with that, but there was no other reason- serving the country, being in the military, being a leader, and etc.
Personally, I do not think it is a fair question to ask a candidate - why they are only applying to
one academy. The implication is that, if you
really wanted to serve your country, you would/should be applying to
all of the academies.
You say "kids will be kids" as if there is something immature about only being interested in attending one academy.
If a candidate is applying to only one academy, say USNA, a
far better question would simply be: "Why do you want to attend the Naval Academy?" If their answer satisfies you, then that should be that. Why complicate matters by probing for why they didn't apply to West Point or USAFA? Hell, for that matter, if it's just a matter of serving, why not ask them why they have not volunteered to join the Peace Corps, serve in government, or become a police officer?
There is ample opportunity for a candidate to go down the wrong path without needlessly complicating the question. In my opinion, you are setting them up to go down the wrong path when they might have not otherwise.
Yet, I maintain that a candidate
should be prepared to answer this question - and you have just proven
why they should be prepared - because the question
does get asked - unfairly, in my opinion.