Problem with text on Angelou stamp?

An interesting issue on copying with improper attribution arises with the Angelou stamp.

From nbcnews:



The stamp includes a quote by Angelou that says, "A bird doesn't sing because it has an answer, it sings because it has a song." The Washington Post reports that there's been some controversy about the origins and attribution of this quote, which appears in Joan Walsh Anglund's 1967 book, "A Cup of Suns."






UPDATE. The Guardian noted on April 8, 2015:

The postal service initially defended itself by saying the quote is associated with Angelou, but when the Washington Post contacted them again after Anglund confirmed she wrote the line, the agency's tenor changed: "Had we known about this issue beforehand, we would have used one of [Angelou's] many other works... The sentence held great meaning for her and she is publicly identified with its popularity."