"...when there is no function of critique or comment on the original work."
Oh, really! I'd say the spoof is dripping with parody! It's "critiquing and commenting" on the state of American society today, using Guthrie's original song as a takeoff point.
The Ludlow people should read the entire set of lyrics for the song, as published in the 1940s:
http://www.geocities.com/Nashville/3448/thisl1.htmlNote that Guthrie is juxtaposing idealized patriotic imagery with harsh, unjust reality. The 1950s version only has the pleasant stuff in it -- itself an interesting commentary on the time, given the McCarthy era -- and I think the "prettified" song actually seems shallow compared to the original. The JibJab spoof is poking fun at the sanitized later lyrics, in particular -- politicians of all types love to invoke patriotism with those kinds of images (especially the Bush administration).
Very postmodern. If people don't mind me going off on a tangent like a typical cultural studies theorist ... in a way, JibJab is trying to return the song to its original roots. They're using a catchy tune and sentimental associations (especially with the 1950s version) to turn people's attention to the shortcomings of both sides, and the whole trend towards polarization and oversimplification in political debates. It's much more faithful to the concept of satire than simple "filking" (writing unrelated new words).
(Oh, and if the JibJab people or their lawyers are reading this and like my argument, I give you full permission to use it.)