Imposing this standard would mean that the publishers would no longer sell to Amazon, Apple, Kobo, Sony and other online-only book retailers. Of course, it would make Barnes & Noble a de facto monopoly for online book sales in the U.S.; publishers may find over time that B&N becomes just as demanding and difficult to work with as Amazon was. On the other hand, it would strengthen independent booksellers, which would no longer have to compete with Amazon.
The ball is in publishers' courts--they can cut off Amazon if they want to. The real question is, why haven't they already done so?