If you read the list, you'll see that at least some of these artists are apparently dead (e.g. Lonnie Donegan, died 4th November 2002; Freddie Garrity, died 20th May 2006). I take it the ability of these dead authors to sign a petition asking for their copyright terms to be extended can only mean that even after death, term extension continues to inspire.
I'm not yet sure how. But I guess I should be a good sport about it, and just confess I was wrong. For if artists can sign petitions after they've died, then why can't they produce new recordings fifty year ago?