This to me is also a security issue. By having the user time out after 15 or so minutes it helps keeps peoples usernames and passwords safer. Instead of getting tricked into logging into a fake BB site and letting someone screw it your team(and stay logged in) they would eventually get logged out and thus would have to wait for you to do it again to login.
I honestly dont mind the log out because its really not that big of a hassle to log back in.