On the case of Girl Talk I think he is  using his rights to MAKE good music. He has "fair use" just in its own instance of him not actually selling his music, instead just putting out for the population to download for free. He wants to produce music, that's all, he didn't even expect to be famous because of it and is just grateful for the opportunity as shown on the video. He explains it perfectly, he's taking an idea/creation of somebody's work and is using it with multiple others to for a single song, Girl Talk is not copying the whole song word-for-word. By creating these songs in his own view, he is creating his tunes through "transformative use." Bottom line in my opinion is until he starts selling albums off of it or bad mouthing the artist to some extent than there is nothing he shouldn't be able to do to express his artistic values.