Stallman tried to introduce the term "intellectual monopoly", which fits better, since they really are monopolies granted by the government for limited periods of time, intended to promote progress in science and the useful arts.

"Property" was chosen specifically as a bait and switch. It tries to get people to take a concept that has been understood for thousands of years for physical objects, and apply it to this novel century-or-two long experiment for encouraging the production of easily-copyable things.