As explained in #422537 it includes files taken from gzip to support
compression. Current maintainer decided to preserve compression support
and so moved the license to GPL. Incidentally it is not compatible with
GDAL copyright, so a practical possibility is disabling FITS support in GDAL.
Another possibility would be re-licensing it as GPL in Debian, but it would imply
licensing all its rdepends in the same way. Is there anyone who does not
agree about that, due to constraints of one of its rdepends? Ideas? AFAIK
licensing under the more 'restrictive' GPL is not a problem due to the
BSD-like nature of the upstream license.