I am not an RIAA lawyer but I suspect that their argument will go like this: In order for iTunes Music Match to find the song it had to be on your disk. In order for it to be on your disk you made a copy of the song without the rights to do so. The act of making that copy was a violation of copyright. You had 10,000 such files.
But then out of the shadows steps your hero Steve Jobs! He offers that with one click you can make all ten thousand copies legal and make the RIAA go away! And it will only cost you $9,900!