Depending on how they serve these clips (do they save them locally etc.) the point could be to take load off the internet archive.

The other point might be to provide better search capabilities than IA, also taking load off. Sometimes verifying something actually is Public Domain on IA can be work in itself, putting more work on a primary resource. In short it seems to me these people are doing a good thing.

BTW they don't hide the source of the videos. Scrolling down on the video page links to the collection and source.