From the article: We are asking New Jersey retailers statewide to stand with us and express their outrage by refusing to carry or sell copies of Coulter's book.
They are not creating a law to ban Coulter's book. They are asking for a boycott.
I know Barnes & Noble's Leonard Riggio is a big Dem donor. It'll be interesting to see what that particular big box does, officially or otherwise. I remember checking with some bookstores after the Swift vets book came out. Some bookstores were just not putting it out, putting in the wrong section, or some employees just plead dumb about having it.
They are representatives of the government asking for a boycott. That is censorship, and as such it is a violation of the Constitution. It would be very different if they were private individuals asking store-owners to boycott the book; that would be fully legal and acceptable.