I don't understand the relevance. The scripture you quoted refers to offerings to other gods. In this case, we're talking about books and movies that don't have anything to do with gods or Christianity.
I guess I don't understand what "issue" we're talking about. Is the argument that someone who's weak in their faith will read a Harry Potter book and be like, "Well, I'm not a Christian anymore"?
I will do my best to explain my thoughts! I always don't reflect my thoughts in mind with appropriate words.. So apologies if I end up confusing you more

Taking this to today's culture.. Watching Harry Potter movies.. Create an opinion poll among Christians.. Ask how many people believe they could be influenced by demons by watching Harry Potter movies.. I am sure you will get overwhelming response.. They are not to blamed as they are not mature in the Christian faith.. And they don't look at it from legalistic perspective.. It's just something that would be in the conscious.. Instead of causing more confusion to their conscious, let's not just watch the movies! Since we are talking about a social issue, when a group of Christians endorse Harry Potter movies, it throws confusion for less mature Christians.. That is why I am saying, as more mature Christians let's not do anything which is not going to glorify God.. I am not saying watching Harry Potter movies makes us any less Christian.. but it does not strengthen the fellowship.. That is all I am saying..