Because you're not a member of the PuTTY core development team. The putty-bugs mailing list is not a general newsgroup-like discussion forum; it's a contact address for the core developers, and an internal mailing list for us to discuss things among ourselves. If we opened it up for everybody to subscribe to, it would turn into something more like a newsgroup and we would be completely overwhelmed by the volume of traffic. It's hard enough to keep up with the list as it is.