Unless it is a status email or email where the party I'm emailing knows the info has to go to the boss (as he may want say) I don't CC, but frankly over the years, I've done this when, frankly I _DON'T_ trust the person!
I've had more than one situation over the gasp 40 YEARS I've been working where you'd tell coworker X, and they would deny you did, and you'd have to jump through hoops to make your manager (and sometimes their manager) believe you
The problem is often solved by "CC:Their Manager, Your Manager"
Gee, you mean it makes the person say "CharlieG doesn't trust me" - well DUH.
I won't do it until the first time I'm under the bus because of you, but after that, I'm trying to not only cover MY ass, but also send the message "No, I DON'T trust you" - not only to you, but to my manager and YOUR manager that I no longer trust you.
BTW, there was also probably an email to my manager (in my case, my 'manager' is the CEO), and your manager about why they were being CC'd (aka how you threw me/the project under the bus) that YOU were not CC'd. When you get to the level I'm at, this is usually about a person who reports to a fellow manager, and the CC/BCC might even be going to the CEO/HR as part of a documentation trail