That might be the excuse they use as "justification" but of course, in fact, there are plenty of reasons for keeping stuff away from snooping eyes. One is it's just private, and/or very personal. Another is it's financial. Another is medical confidentiality. Yet another is company/commercial secrecy. And yet another is that if the message involves the transmission of personal information, the the Data Protection Act might actually require steps be taken to protect it.
And I'm sure, broad as those categories are, it's not an exhaustive list.