I agree that it could be confusing, but so would the absence of any invitation to reply. What would you suggest as an alternative?
For someone not familiar with the site, it really might not be obvious that they can respond at all, let alone respond without becoming a "member". The presence of a "reply" link signals that this is an active conversation to which people can still participate. The usual signal on the web of a members only site/closed discusssion is listings of posts without a "You can reply button".
Maybe change the label to "Discuss"? That would invite without necessarily implying that you are going to enter a reply immediately. I think people would not be surprised to see "the discussion" instead of a reply box if the link was labeled "Discuss".