The Fair Use Doctrine, copied and pasted below from the US Copyright Office stipulates that it is legal and permissible to use copyrighted material under the fair use doctrine for purposes of parody.
Copyright experts we advised with before posting the song told us in no uncertain terms that we were within our rights to use the song because we did so in accordance with the Fair Use Doctrine.
Regarding Napster's claim that its users were not infringing copyright because sharing copyrighted musical compositions and sound recordings via the Napster service is protected by the "fair use" doctrine, the court held that the four fair use factors in the Copyright Act tip the balance in the record companies' favour.
David Cameron pointed to the " fair use" doctrine in American law, which he said had allowed companies like Google to flourish without fear of infringing copyright.
When it is impracticable to obtain permission, use of copyrighted material should be avoided unless the doctrine of fair use would clearly apply to the situation.