Personally, my guess would be that ffmpeg may be too influential. Banning
ffmpeg for patent-reasons would be pretty much poking not the bear, but the
bee-hive of pissed of techno-geeks everywhere.
As the mail states, there are intentional exceptions for low-volume (in
business speak, roughly the same as low-income) "in order to encourage
adoption". I would assume that for peace:s sake, MPEG LA is avoiding
confrontation to much provide the patent opposition with too much fuel for
their fire, and further de-rail wide-spread adoption.