Taken from the copyright notice on the back of an Aerosmith CD:

Unauthorized reproduction, copying, and rental of this recording is prohibited by law.
If that notice instead said that unauthorized reproduction, yada yada yada, was *not* prohibited, then and only then would it be legal, unless you have authorization to do so. Put simply, it is illegal to do anything with a recording that you are not authorized to do. Making a copy on MP3 (or making a copy in any other format) IS illegal. But that doesn't mean you'll get caught or even that the RIAA even cares to prosecute you for it.

By copying that recording, you would be violating that agreement, which would also be a violation of the copyright act, a federal law in most countries. That makes it a criminal offense.

Simply because you will not be prosecuted for that offense does not make it legal.