Since banned accounts are not revealed to forum members as banned, it won't be anyone's faulty if they merited such posts as that of a banned user. The quality of the post could be an attribute that made it irresistible for a member to award some merits to the post. It doesn't change anything, but if a user is aware that a specific account has been removed from the forum, it's a bad stewardship to waste merits on their posts. Hence, the forum doesn't have any penalty for that, if not, they would have disabled the merit button on any post associated to a banned account.
If we already know that account will already banned then for sure we won't give them merits because of the reason that it will waste the merit circulation. But we can not deny the fact that before those user being banned they make a good or a quality post or let say they have done a good work that contribute on the forum. Which is worthy enough to give some merits and this is what happens why some banned account still got merits even though they are banned.