Sure you bought it, but what did you buy? Did you buy the rights to whatever you please with it? It depends. Say I only sell an MP3 through Amazon what are you doing? You're buying it with the following terms of use, which you agree to by the very act of forking money over for it:

2.1 Rights Granted. Upon payment for Music Content, we grant you a non-exclusive, non-transferable right to use the Music Content only for your personal, non-commercial, entertainment use, subject to the Agreement.

2.2 Restrictions. You must comply with all applicable copyright and other laws in your use of the Music Content. Except as set forth in Section 2.1 above, you may not redistribute, transmit, assign, sell, broadcast, rent, share, lend, modify, adapt, edit, license or otherwise transfer or use the Music Content. We do not grant you any synchronization, public performance, promotional use, commercial sale, resale, reproduction or distribution rights for the Music Content. As required by our Music Content providers, Music Content is available only to customers located in the United States.