First off, not a lawyer.
That said, as long as our links to ANY site aren't breaking the TOS for that site, I don't see why we shouldn't allow them. Linking Wikipedia or Pottermore isn't any different than linking YouTube. As you can clearly see in the YouTube TOS, linking or embedding on a SE page meets all the qualifications set forth in section 4 & 5.
I agree that we don't want to get into a copyright pissing contest, but if we (SE) aren't hosting the content, it's not our problem that it's available on the open web. As long as links are clearly stated to lead outside the SE network, then they should be allowed to stand. Additionally, why should we be writing, when a few second of video can clear the confusion right up. We do Q & A here, lets not tie our hands to jump through hoops we don't have to.
Additionally, our own sites legal page already has a disclaimer that seems to directly reference this:
- Third party websites
Users of the Network may gain access from the Network to third party sites on the Internet through hypertext or other computer links on the Network. Third party sites are not within the supervision or control of Stack Exchange or the Network. Unless explicitly otherwise provided, neither Stack Exchange nor the Network make any representation or warranty whatsoever about any third party site that is linked to the Network, or endorse the products or services offered on such site. Stack Exchange and the Network disclaim: (a) all responsibility and liability for content on third party websites and (b) any representations or warranties as to the security of any information (including, without limitation, credit card and other personal information) You might be requested to give any third party, and You hereby irrevocably waive any claim against the Network or Stack Exchange with respect to such sites and third party content.