To put it simply: You are wrong.
Theft is taking something without the owner's permission whereby you deny the owner the use of that thing. Copying a thing is not, therefor, theft, for the thing still remains in the owners possession. For example: I copy the Mona Lisa. The original is still there on display, but now I have a copy. I copy a track from a CD: The CD is still in the possession of the owner, but I now have a copy. If I download a copy you made of a CD, I have a copy, but the CD is still in the hands of the owner. I do no take possession of the original, and I do not take possession of the copyright.
Copying the music without permission is, however, breaching the rights of the copyright holder, and for that they can choose to pursue me and bring a private, civil prosecution against me.
Copying the music and distributing it is facilitating and encouraging the breach of the copyright holder's rights, and opens me up to a more severe penalty. But it's still a civil offense. One the copyright holders will want a lot of compensation for, but it's still in the land of civil offenses.
Selling copies of the music is criminal: It's fraud. Why? Because I'm implying I have the right to sell the music when I don't. Still not theft, though. Not unless I stole the original to copy it...
The only time you could claim someone has stolen copyright is if that is what they've done: Taken the copyright away from the owner and claimed it as their own. But please note before you argue that's what copying is doing: It isn't as the owner of the copyright still has the copyright and can still enforce it. To steal the copyright is therefor somewhat difficult.
However, it's more intimidating to say 'copyright violations are theft: You can be arrested and locked away for ten years!'. It even sounds plausible to people who do not understand the law.