It is tempting to believe that people who don't know the difference between reply and reply to all are stupid. 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.