Artists don’t seem all that clear themselves when they discuss their relationship to copyright and fair use. For example in Print magazine, Milton Glaser says, "...For myself—this is subjective—I find the relationship between Fairey’s work and his sources discomforting. Nothing substantial has been added. In my own case, when I did the Dylan poster, I acknowledged using Duchamp’s profile as an influence... " Look at the pictures below and see why this raises the gray areas of fair use, plagiarism, referencing, and appropriation. The little block on the upper right of the famous Dylan poster says "Milton Glaser." Yup, I didn't see Duchamp's or Peter Max's name either.
Perhaps we should look to the old phrases guide us; "Let your conscience be your guide," "Handsome is as handsome does," "Do unto others as you would have them do unto you." Or maybe we just want to have our cake and eat it too.