You're confusing a figure of speech for a literal description. Book addiction is seen as good because nobody is talking about literal addiction, when it comes to books. They're talking about a great enthusiasm, that is both healthy and beneficial for the enthusiast. Nobody thinks a literal addiction to books is a good thing. Nobody says that.

I'm curious: Do you find that you struggle with literal versus figurative meanings generally, or is it just that this particular figure of speech slipped past you somehow?