It's extra rude if you consider the history. Bookstores of old had a counter where you entered, you placed your order and if the book was in stock someone would go and get it (or in case you ordered ahead they'd get your pre-packaged order for you). Then we figured that self-service is the way to go: rely on the customer to do their own orderpicking, en-passent exposing them to a lot of marketing and hopefully selling some more books.

So now, on top of being required to do the work of the bookstore employees you also lose the benefit of the doubt and are being treated as though you are a common thief.

Industry secret: lots of theft from bookstores is by employees.