I can understand terry pratchett books not being in kid area (maybe too difficult reading wise for small kids?) But teen area should be totally okay. Here the library divide the teen more closer to adult section than to little kids section, teens too cool being seen together with the small kids

Well, they have way more adult themed media there (eg. the aforementioned Blacksad, or The Witcher games & media). That's the striking contrast I'm trying to point at.