My kids are grown up now so I've never heard of Deary. But it seems to me he's missing the point of libraries - afaik they are all about promoting literacy as a social good. In the wider view, as an author he stands to benefit - more literacy; more demand for books in general.
There's also the indirect benefit to him personally. If parents become familiar with his books they are more apt to buy them - it's kind of like free advertising, or an introductory special.
Deary seems like a narrow-minded sourpuss to me with a very self-serving view of the world. Literacy doesn't exist to serve his purpose; he's privileged to have a gift for writing and honoured to have many adoring readers. He's making a contribution to society and co-incidentally making a good living. So, what's he whining about?