[DNSB]

I suspect this is from a strict adherence to life+70. Since Dame Christie passed away in 1976, her works would not be available until 2047 despite many of them being covered under older copyright rules which required renewal or had shorter terms. It seems that many people forget that in the US which seems to be where most of this fuss comes from, the life+70 protection only applies to works created on of after 1978-Jan-01 but other countries extended the life+70 protection to any works created by an author. That had the somewhat humourous side effect of books that were out of copyright going back into copyright.