Actually, the itunes music store uses AAC encodes, not MP3. There is a way to remove the encryption and convert it to a normal, non-encrypted AAC file, which I've mentioned in another thread about online music purchasing sites.
I'm of the opinion that if you're going to purchase something you should have the right to do what you want with it. It's one of the reasons I hate seeing commercials (not previews, commercials) before a movie.