That’s not at all how I read the post you’re referring to. The earlier post says, “People link to videos all the time, in fact. And they even link to videos that violate copyright, which happens all the time on youtube, although your link does not appear to be an example of it.” But no mention of not linking to videos that violate copyright. (Which is how it should be, I think. Generally, we cannot judge if a video violates copyright.)
There was a thread in CS (or maybe MPSIMS) wherein Tuba (I think) or Lynn closed a “share cool Youtube links” saying that the SDMB wouldn’t indulge in copyright violations. And at that point, Youtube was really new.
I can’t find it either, FWIW. The thread may have had Opal in it.
Well that narrows in down. 
This is the closest that I could find (March 2006)
http://boards.straightdope.com/sdmb/showthread.php?t=362679&highlight=YouTube
The first mention of YouTube here was a month earlier; a zit popping video.
That thread wasn’t about sharing YouTube links though. It was about downloading and saving them to your computer. The latter may have been thought a copyright violation by some. My point of view is it’s up to copyright holders to notify YouTube of potential copyright violations and ask them to remove it. It shouldn’t be up to the casual user to determine if it’s okay to download it. If it’s posted on YouTube, I assume it’s okay to download it.