Various holy books most certainly do contain values and norms. 10 commandments. Love thy neighbor. Etc.
People extract the ones that they like, certainly, but what they like could be as simple as what they've been taught - which is to take the values and norms in their holy book literally.
What Aslan attempts to do is absolve religion and religious teachings entirely from bad things, because there are many believers who want to hear his message. It keeps them from having to question religious faith in general (and thus their own). But his position is mindlessly easy to demolish - as long as a moral commandment or teaching is in a holy book, it's possible for someone to extract it. Good or bad.
On edit: I wanted to note your question - "what purpose does scripture serve then?" - is an excellent one and I would love to hear him try to answer it. 
Edit history
Recommendations
0 members have recommended this reply (displayed in chronological order):
= new reply since forum marked as read