Bookshops are reluctant to buy books they aren't sure of: the worry is that they'll get stuck with stock they cannot sell, and so lose profit in an already tight market. Consequently, booksellers try to avoid buying any stock on a firm sale basis, and usually do all they can to buy books on sale or return only.
Despite the valuable breathing room that returns provide to booksellers (particularly to the independent booksellers), returns cause all sorts of problems. Sales reps are reluctant to authorise them, because doing so takes up valuable selling time; publishers don't like them because returned books are shelfworn, cost them money, and yet can't be resold; and writers don't like them because they lead to nasty "reserve against returns" clauses in their contracts which allow their publishers to hold back a percentage of their royalties in order to allow for any returns.
Emma, over at the rather wonderful Snowbooks, wrote a couple of excellent blog posts earlier on this year in which she discussed the many problems that returns create. I can't better them: you'll find them here and here.