Frequently, those copies are fraudulently written off as destroyed at the cost of the publisher; the publisher didn't make their money off the sale.

> If these promotional copies already being sold by other bookstores then what's the problem?

Being sold in a mom&pop bookstore is a very different case than being sold through the biggest internet retailer.

> If they shouldn't be sold then the publishers should clearly mark them as not for sale.

Sometimes it's the authors who get the promotional copies and attempt to give them out to fans or friendly distributors as a method of marketing; some of whom are dishonest about their reasons for asking for promotional copies.