GNU Emacs is licensed under the GNU General Public License (GPL). The GPL is a copyleft license that requires any derivative work to also be GPL-licensed. Does that impose restrictions on what license can be chosen for third-party Emacs packages such as those found in MELPA and several other repositories around the internet?
I believe the answer to this question depends on the interpretation of the legal term derivative work: if a third-party package is a derivative work of GNU Emacs then the package must be GPL, else it can use another license. (Similar concerns have historically arisen with dynamically linked libraries.) I didn't find any clearly stated opinion on this matter on EmacsWiki, the Emacs Lisp reference manual or the GNU website.
In practice, we have third-party packages bearing e.g. the MIT license notice, but I wonder whether that license notice has legal clout and the real license ought to be GPL anyway.