Yes, it is as designed that way (on purpose). The associated discussion thread does not get Closed (locked) either. Discussion is the entire point of Forum Communities. Post game discussions can go on for days, weeks, months and sometimes even years. I have several events from previous NFL seasons that members are still commenting on. A lot of sports communities that use Sportsbook have more ongoing POST GAME discussion in Sportsbook comments than they do in discussion threads.
With that said, one of the first new functions added to Sportsbook 2.0 was a "Lock comments" function. This is a popular feature that I implemented several years ago for Showcase, AMS and UBS and decided to include this function with all XF2 versions of my addons that have a comments system (including Sportsbook, Power Rankings, Pickem/Survivor, PRS etc). This will allow you to lock/unlock the comments section on any event, at any time.
This is a permissions driven function. There are 2 permissions. One is a bookie permission that allows the owner of the event to lock comments in their own events (but not others events). The other is an event moderator permission that allows event moderators to lock comments on any event. If the viewing user has permission, to lock/unlock comments, they will see the Lock comments link in the Event Tools (if comments on the event are locked, the link will change to Unlock comments).
Note: Sportsbook Bets have this function as well and they have their own set of permissions.