NetworkManager in Wheezy resets MAC address to the inital one once disconnected
See GNOME bug 708820
Even once this is fixed upstream (for Jessie), we might want to simply comment out the resetting code in NM (as long as Tails is based on Wheezy). After a quick look at the code, it seems to be a one line change in two places.
Parent Task: #6015 (closed)