Yes, I agree that this is an irritating feature of the BBS and I would like to see it changed.

Note that it doesn't actually forget anything, it just assumes that if you enter the forum index from a top-level forum link after not having come up from a thread, then you want all the old posts from before your last visit marked as unread. It's kind of complicated how it works under the hood, and I wish it could be changed or made an option.