MP3 files in the Amazon MP3 Music Service are free from DRM restrictions; however, the Amazon MP3 Music Service: Terms of Use agreement imposes legal restrictions that customers should be aware of and compare to their rights under the first sale doctrine with a CD purchase.
In section 2.1, it states:
Upon your payment of our fees for Digital Content, we grant you a non-exclusive, non-transferable license to use the Digital Content for your personal, non-commercial, entertainment use, subject to and in accordance with the terms of this Agreement. You may copy, store, transfer and burn the Digital Content only for your personal, non-commercial, entertainment use.
In section 2.2, it states (excerpt; italics added):
Except as set forth in Section 2.1 above, you agree that you will not redistribute, transmit, assign, sell, broadcast, rent, share, lend, modify, adapt, edit, sub-license or otherwise transfer or use the Digital Content. You are not granted any synchronization, public performance, promotional use, commercial sale, resale, reproduction or distribution rights for the Digital Content.
Source: Dudley, Brier. "Unlocked Music Isn't Unlimited." The Seattle Times, 8 October 2007.