And the Bill of Rights, like much of American law, was either based on English law, or based on things the English were doing that Americans didn’t much like. So, respectfully, cricetus’s point directly responded to the OP:
Americans answer that, yes, it is – so fundamental, in fact, that for us it ranks right up there with freedom of speech, the right to be secure in our homes from governmental intrusion, and the right to a trial by a jury of our peers.
Cross examination is considered by many to be the greatest tool for discovering the truth that Western civilization has developed. In the old days, trials were simply gamesmanship: neither side told the other much of anything, and all was revealed (Perry Mason-like) at trial. For all I know, English trials are still run that way.
In America, however, we have decided that getting to the truth comes an awful lot better if both sides know (or at least have a general idea) about the evidence to be marshalled. As don’t ask put it, if I don’t know who the witness is, I don’t know if they have a dog in this fight. In the Michael Jackson trial, for example, is it relevant that the accuser’s family have allegedly attempted to obtain money under false pretenses from other celebrities and businesses? Yes. Could the defense team have found that out if they didn’t know who the accuser was? Perhaps. Perhaps not.
My take on it? It’s a form of evidence preclusion – if a party misbehaves badly enough, the court can sanction that party by precluding it from presenting certain evidence (for example, the court could limit or eliminate cross-examination of a prosecution witness). But the article doesn’t state whether the defendant in fact tampered with witnesses so as to justify preclusion – it simply seems that everyone assumes he did.
Either way, to answer the OP: yes, the right to confront the witnesses against you in a criminal trial is a fundamental element of due process. It may not be a human right, but it comes darn close.