I did not know Schelter, and have no idea what his opinion on free/open
software was. I respect his contributions, but let's keep the history in
mind.
The basic reason that the 1982 version of Macsyma (DOE Macsyma), which
remains the core of Maxima, is and should be available to all without
restriction is that (a) it was developed by MIT (and others) under U.S.
Government funding; (b) until a fairly late date, it was not copyrighted by
MIT (in fact, during most of its development, it was generally believed that
software could not be copyrighted). Schelter, Fateman, and others maintained
this non-proprietary version in parallel to the commercial, proprietary-code
Macsyma.
Both the concepts of "free" and "open" software postdate 1982, and anyway
only Schelter's and the community's modifications are restricted by GPL.
As a statement of current policy, avoiding ideology, I think it's fair to
call Maxima "free, open-source software" or some similar compound.
-s