Having the license notice of each file match the licenses of the final
binaries reduces confusion. Otherwise one has to know how the code is
linked together, and the licenses involved, in order to explain the
dichotomy. It also makes it easy to borrow GPLv3+ and LGPLv3+ licensed
code from GEGL and GIMP.

Commit 78ea329f explains why the final binaries must be under
GNU General Public License version 3 or later.