As I understand it the tags are entered both by Amazon and by publishers, and it's entirely possible that the publishers wanted 'Adult' on the titles to differentiate them from 'Childrens' and 'Teens'.

Certainly, in the UK, that was how public libraries classifed things, and book publishers are a traditional lot.

And I very much doubt that the Ledding Library in Milwaukie, OR has 100 000 items of porn.