Emacs, in its incarnation as GNU Emacs, along with GCC, was one of the cornerstones of the GNU project, whose aim was to create a free Unix-like operating system. In 1985 GNU Emacs was licensed under the GNU Emacs License. The GNU Emacs License was the world's first copyleft license, and was eventually generalized to become what we know today as the GNU General Public License Version 1.
From Free as in Freedom Chapter 9:
Encouraging others to adopt the same licensing practices meant closing off the escape hatch that had allowed privately owned versions of Emacs to emerge. To close that escape hatch, Stallman and his free software colleagues came up with a solution: users would be free to modify GNU Emacs just so long as they published their modifications. In addition, the resulting "derivative" works would also have carry the same GNU Emacs License.
The revolutionary nature of this final condition would take a while to sink in. At the time, Fischer says, he simply viewed the GNU Emacs License as a simple contract. It put a price tag on GNU Emacs' use. Instead of money, Stallman was charging users access to their own later modifications. That said, Fischer does remember the contract terms as unique.
"I think asking other people to accept the price was, if not unique, highly unusual at that time," he says.