[b409ba0801cd21]

The copyright holder of a program can legally license their program under the terms of the GPL along with extra terms of their choosing. Of course, the program's overall license may be incompatible with the plain GPL, and the program's adoption may be hindered.

The fields of endeavor stuff is from the Debian Free Software Guidelines and similar documents.

[jahewson]

The GPL strictly dictates what kind of "extra terms" are allowed. If the author attempts to add non-allowed terms then the GPL itself is invalidated, according to its own statements.