“The trick to this magic is to simply open source the parts you
are forced to by the license and keep the additional code you have
developed secret. Android is issued under the Apache License which
allows modification and reissue under another license. The software
that Amazon released is covered by the GPL and LGPL licences and it
is a condition of these licence that the source is reissued under
the same license – i.e they are copyleft licences.“According the FSF, the source code Amazon has released doesn’t
include any of of the modifications made to create the Fire
operating system. In particular there are no deals of the user
interface or the DRM.”