What's the big deal about cross-posting?

Because VenicePlace is a private, invitation-only list, it is 
important that you do not cross-post or CC: any messages from 
veniceplace to any other list, even other slash lists.  Often, 
this allows the veniceplace address to be seen by members on the 
other lists, which negates VP's privacy.  It is perfectly all 
right to CC: another member of the list, and some software 
programs do this automatically when replying to messages.