Well, if the uploader's using copyrighted recordings rather than a midi or performing the pieces themself or using a more liberally licensed recording, it is understandable. Bach's pieces may be free but musicians' performances of them aren't.
I only listen to classical music, and I've noticed that where YT used to once be a great source of legally-free brilliant performances (often with scores!) it's been going downhill and I've slowly been redirected to copyrighted Big Media recordings of old classics (which usually I do not prefer).
This was already answered by klyrs further up the thread: because the videos are subject to YouTube's policies which commonly involve altering or destroying parts of the media without warning.
And, fwiw, the remainder of the old movies she put up didn't get flagged, so you're wrong about the fitness of the platform for dorky family videos.
And yeah, the remainder of the videos might not have been flagged, but we're talking about the one that did. Please keep the focus placed on the problem that you brought up.
While I agree that it lacks some context, it seems like smaller mislabeled clips were the source of this "violation" rather than the usage of whole copyrighted clips.
It's kind of nonsensical that performances based on a public domain piece of art can be copyrighted. That would be the similar to making the Linux kernel proprietary again just because you change the way the code is formatted.
Also, let's not even pretend it's impossible to imitate someone's way of playing while still doing your own interpretation. How would algos know?!?