I think we’re getting crossed here. First off, I’m not discussing purchased works since it is obvious that when a creator puts their work up for sale to be used in other works its use is implicit in the copyright agreement. Copyright comes into existence the instant a work is created and that copyright is vested to the work’s author. It is in the transference of that copyright that holds the conditions of the work’s use. When I sell an art piece I draw up a contract of copyright transference in which I set the conditions under which that piece will be used, reproduced and to what extent amongst other things like payment and royalties depending on final use. The copyright remains with me as its author (creator) and if the conditions of that agreement are not met I can pursue legal avenues.

When artists decide to put their works up for free or in the public domain they place conditions of that transfer whether it be not for commercial use or to be used freely in any way or for a simple credit mention back to the author. These are conditions of copyright transference. So if an artist asks for credit mention why would we repay their kindness and hard work by begrudging and disrespecting the conditions of the transfer? We are quick to grab the free labor of others but too lazy to give them mention for their generosity when we are praised for our great artistic endeavors when using their work.

The question of Shepard Fairey and Mannie Garcia is one of derivative works. Where one work is created from a prior preexisting work and is covered under U.S.C. Title 17 Chapter 1 §103(b):

“The copyright in a compilation or derivative work extends only to the material contributed by the author of such work, as distinguished from the preexisting material employed in the work, and does not imply any exclusive right in the preexisting material. The copyright in such work is independent of, and does not affect or enlarge the scope, duration, ownership, or subsistence of, any copyright protection in the preexisting material.”

 Just like "The Westside story" and "Romeo and Juliet"