You must be logged in order to post messages. If that weren't the case, anyone could post messages without first registering for the BBS.
You were logged on for all of your previous postings. The proof is that the BBS would have no way of determining that "cheter" should be credited with the posts if you weren't logged in when posting.
Will you have to log in every time you access the BBS? Not necessarily. When you log in, there is a box with a question that says, "Remember me?" If you check that box, you will be automatically logged in each time you return to the BBS when using the same PC.
Why did you have to log in this particular time? That happens if you access the BBS from a different PC than you normally use. It also can happen if you deleted (intentionally or accidentally) your PC's cookies for the BBS log-in.