Junk and Trash are both virtual folders that "peek" at the contents of
other (physical) folders on the same store. Thus, when you expunge a
Trash folder (or a Junk folder), it actually goes and expunges *all*
the folders in the same store.

This is not a bug.

One could argue that a person MAY want to retrieve an item from the
trash, but probably never from junk. Thus, when expunging Junk, a user
could inadvertently lose a mail item he/she wanted that was in trash.
The warning message is not complete, however. It reads:

This operation will permanently remove all deleted messages in the
folder `Junk'. If you continue, you will not be able to recover these
messages.

It does not mention Trash at all.

Regards