I regard it as a breach of etiquette at best. At worst—well, I ended a friendship over it. If I had wanted X to see everything I had to say about subject Y, I’d have included him in the first place. I protested to the individual concerned, and he flatly refused to acknowledge my point. So no more correspondence from me, ever.
I well understand that there’s no assurance of privacy when it comes to e-mail, or indeed any other kind of correspondence or communication; but I still think courtesy demands asking the other party or parties if it’s ok to add someone or forward to someone, unless it’s a clear-cut case of need to know in a business or official context.
Even then, it often seems best to truncate the message to the relevant parts instead of including an entire thread that may have other content. How hard is it to ask oneself, “Would X really be fine with my sharing Y with Z?” If there’s any doubt, don’t.
Published is published; you can’t take anything back.
Of course, there’s always BCC, which might occur on any message you receive, and you wouldn’t know it. If there isn’t a little trust between correspondents, though, it becomes something other than good-faith communication.