Can anyone tell me why an RSP uses dynamic proxies?

Vodafone apparently uses them on their RBI service.  The consequence is that two sites I use regularly give me problems.  One is a webmail server that checks the user's IP address and, for security reasons log me off when it sees a different IP.  It does this many times in a session.

The fix for this issue is to change the config file for the webmail server.  But that is a commercial server and even if I had access to it, such a change would not be permitted.