[License-discuss] Why GPLv3 does not have a indemnification clause by default?

[Patrick Schleizer]

Quote GPLv3:
>   Notwithstanding any other provision of this License, for material you
> add to a covered work, you may (if authorized by the copyright holders of
> that material) supplement the terms of this License with terms:
> [...]
> 
>     f) Requiring indemnification of licensors and authors of that
>     material by anyone who conveys the material (or modified versions of
>     it) with contractual assumptions of liability to the recipient, for
>     any liability that these contractual assumptions directly impose on
> those licensors and authors.

I wounder why GPLv3 allows to supplement terms related to
indemnification, but didn't add such an indemnification clause by default?

cc'd public license-discuss and whonix-devel mailing list so everyone
can benefit from your reply.

Kind regards,
Patrick