I don't know if this is intentional but other softwares such as IPB do show added users the full PM history.
WhatsApp, for one, behaves in the same way.
CCing new people in a chain of emails lets them see all of the emails.
It seems there is no real standard on this behaviour, although I will say this... _not_ knowing is not necessarily better either.
Some notification to the end user should be present, like a help tooltip or otherwise...
This was by design.
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CCing new people in a chain of emails lets them see all of the emails.