While it’s often (usually?) possible to change this via the software driver, nobody thought of it as a big deal.
The TAILS distro's, since V1.0 (which was released in May 2014), includes the functionality referred to as "MAC address spoofing"... "How to spoof the MAC address The first three bytes of a MAC address determine the Organizationally Unique Identifier (OUI) which in practice determines the chipset's manufacturer. The current implementation of [MAC address spoofing] leaves the OUI part unchanged, and only spoofs the last three bytes of any network device's MAC address immediately after it is added by udev." Three bytes is 2^24 bits, so that allows your computer to be randomly assigned one of about 16 million MAC addresses each time you boot up TAILS. More details here cheers, Ian.