Not necesarilly true; it would seem that many bookstores are approaching manga (and other graphic novels) with the same wary trepidation as many comic book stores (but for perhaps different reasons), as evident from this post by UCDavis Bookstore's Paul Takushi, which reads in part:
I must admit that I resisted at first. I heard stories from other booksellers about how the books get trashed and picked-over but not actually bought, how GN readers are notorious shoplifters, and how GN readers like to hide books in other sections so they can "reserve" it for themselves while they read it inside of the store.
Well, the section does get picked over, but not exactly trashed. That's a good thing because it means that people ARE looking at the books. The sales continue to increase while I fine tune the selection and the word gets out that we stock them. Since we created the section, no books have ever been stolen.
The reasons may be different, but the root cause is the same, in that some are reticent to go outside of their 'comfort zone' in merchandise: floppies for comic stores, 'normal' books for booksellers. But when there's an audience with dollars to spend, commerce will find a way. Those stores that recognize this emerging market, and cater to it, will undoubtedly find it profitable.