Licensing

[swsnr]

Shouldn't this mode be licensed under the GPL?

I am no lawyer, but I think, this mode might constitute a “work based on the Program”, that is, Emacs, according to section 0 of the GPL 3, since it makes heavy use of Emacs' built-in functions.

To “modify” a work means to copy from or adapt all or part of the work in a fashion requiring copyright permission, other than the making of an exact copy. The resulting work is called a “modified version” of the earlier work or a work “based on” the earlier work.

Hence, it would be subject to section 5:

You may convey a work based on the Program, or the modifications to produce it from the Program, in the form of source code under the terms of section 4, provided that you also meet all of these conditions:

[…]
c) You must license the entire work, as a whole, under this License to anyone who comes into possession of a copy. This License will therefore apply, along with any applicable section 7 additional terms, to the whole of the work, and all its parts, regardless of how they are packaged. This License gives no permission to license the work in any other way, but it does not invalidate such permission if you have separately received it.
[…]

Emphasis and omission mine.