I was asking this question myself recently before I redistributed the Standard Rules. As of 2016-02-04, the Inform 7 website had the text @ArdiMaster quoted above:
The Standard Rules, used in every Inform 7 project; they create the basic assumptions about the model world, and define our language for talking about it […]
Licence: Copyright 2006-2009 by Graham Nelson; published under the Artistic License 2.0 […]
Published: April 2008.
Version: 2/090402.
This was subsequent to the release of 6M62 in December 2015. I’d have felt much better if the listed dates and version were up to date, but I concluded the intent seemed clear enough to cite the Artistic License 2.0 and proceed.
But that’s about the source code. People distributing a game aren’t distributing a copy of the Standard Rules source, the thing to which that license applies, they’re distributing a binary object whose content is derivative of the Standard Rules, and English Language by Graham Nelson (also included in every game but it uses authorial modesty so is less obvious), and the I6 Templates, etc. The bit @Natrium729 quotes make clear that no proprietary interest in the output is claimed.
(Edit: well, one proprietary interest: the requirement of a banner naming Inform 7 and the versions of the compiler and library.)
(Edited again: nope, that was wrong, the Artistic License 2.0 does have implications for compiled versions; see below.)