If you reply to a thread you're probably going to stay logged in in case you get a reply back. In the case that you don't end up posting anything else for the day, your last online status could become an indicator of when you usually stop browsing. Also the general idea of private activity tracking leaves a bad taste in my mouth. Yes, you could log in and out each time you reply but I figured a forum setting would be easier if enough people are also privacy minded.