Shlup says young teens shouldn't read them and that means the author wasn't attempting to write a book for young teens? I don't understand.

Whether she was or not, that's where it ended up. The Da Vinci Code is written about as poorly, with horrid descriptions on all levels and Goosebumps-esque 'cliff-hangers' at the end of too many chapters, but that's a book marketed at adults. And I think that's probably the clincher there; marketing. I don't think too many authors (especially new, unknown and unestablished writers) have very much say on how the book is marketed or who it should be aimed at; they'll just be happy it's selling.

There are many many poorly written books made for adults, and even more made for kids.