Copyright laws are in existence so that someone can’t take someone else’s work and pretend that it is their own, and get money or recognition from it. This is not what Danger Mouse did. When remixing songs I think it becomes copyright infringement when some one can listen to that song and not feel like they need to get the original song because so much of it is in the remix. I feel like that could be the case with Jay-Z and The Grey Album, but Jay-Z encouraged this. No one is going to listen to the The Grey Album and think “Oh sweet, now I don’t have to go get the The White Album because this sounds so much like it.” What I do think is copyright infringement is Coldplay ripping off Joe Satriani with their song Viva la Vida.
The music industry is trying to stifle technologies for several reasons, the main one of course being economic. With the technology that is out there a kid in his bedroom can do what many people in the music industry do for a living, at which point the kid releases his into the world for free, and several people are out of a job. Personally, I love remixes and I don’t think it takes away from the original. To put it in other terms: If someone found out that if you overlapped a Picasso painting and a Michelangelo painting it made something awesome, you would still give credit for the two originals, but you would also give credit to the person who combined them.