[FabHK]

It seems to me that the general paradigm is that the copy/paste operation copies over everything, if the receiving app supports it, and degrades gracefully, otherwise. In other words: copy over as much as possible.

Mail supports rich text, so that's what's copied.

Agreed though: in the scenario you describe, it's often not what's desired, but that's why there's another key combination to support this use case without breaking the consistency of the general paradigm.