It is tempting to believe that
people who don't know the difference between reply and reply to all
However, I saw an example of this a couple
of years ago, on a mailing list
that casts doubt on this theory.
What happened was that the person
who replied to the list wanted to mail the sender
to discuss the arrangements
for the upcoming PhD oral examination of a graduate student
Accidentally, he mailed
the entire list with comments about the student that should
definitely have been kept private.
Was the sender stupid? Well, he does have a
Fields medal (the equivalent in mathematics
of a Nobel prize)
so I'd have to say no, just not particularly focused on email.