As I pointed out earlier in the thread, any electronic communication that isn't encrypted will be read by a 3rd party at some point. Assuming otherwise is like thinking the message on the postcard you sent Grandma is private, too.
There is a difference between avoiding unauthorized interception of traffic and an ongoing policy of authorized interception. The former you assume to happen, the latter should be very clearly made known.
Luckily, being a member is voluntary and no one forces anyone to join and sent PMs.