But what about the lurker? Say someone does just what you've mentioned. Now, back on the Forum Index Page, he/she wants to look at other Forum Topics Pages that he/she has not viewed that session, and has no intention of going back to any previously viewed Forum. If the folders remained yellow just because no threads were opened, the lurker doesn't have a visual clue about which ones have already been viewed. You're asking for it not to notice you've opened a Forum. It would be my hypothetical lurker's desire that it does notice.
The software is not set up to adjust to someone who might want to take a peek and come back after looking in on other forums to see if something more interesting is there and should be read first. It seems better the way it is than the way you suggest. Every action has a consequence. Now, if there was some "reverse action" or "nevermind button" that one could intentionally invoke, that would get around the dilemma for you, while still serving the needs of others with the opposite bent.