Past mailings are deleted if group mailed to is deleted

Hi,
If you delete a mailing list group than all of the "sent" CiviMailings that included that group are also deleted. this results in loss of mailing history/archival information which is very important to the people doing the mailings.

The posts related to this issue can be found here: http://forum.civicrm.org/index.php/topic,13498.new.html#new.

Piotr suggested filing an issue so here it is.

Thank you all so much,
Cynthia