I expect that "who" is used because a copyright holder can be a person, and there is no deeper meaning to it.
To expand, YouTube handles copyright disputes automatically, and it probably wasn't considered worth it to have the system use different text depending on whether the copyright holder was a person or not. They would have had to have asked the copyright holder whether they are a person at some point, and they would have to store that information. It would be additional work for no real gain.
YouTube is an American company, and as far as I'm aware using "which" is preferred over "who" in American English, but "who" was probably considered correct enough – it will be understood, it works for copyright holders who are people, and it doesn't sound too odd for copyright holders that aren't people. Consider:
This video contains content from Jane Doe, which has blocked it on copyright
In British English, "who" would be the pronoun I expect to hear and, as @choster says, it is perfectly correct to use "who." In British English, it's also common to refer to companies as plural rather than singular:
This video contains content from BBC Worldwide, who have blocked it
on copyright grounds.