"Why Copyleft?" is a short essay in which stallman explains the strategic and ethical rationale for copyleft as a licensing approach. It addresses the question of why, given that the goal is software freedom, stallman chose not simply to release GNU software into the public domain or under a permissive license that allows anyone to do anything.
The answer is two-part. The strategic argument is that copyleft — as implemented in gpl-v1, gpl-v2, and later gpl-v3 — prevents the reappropriation of free software into proprietary software. If GNU code were released under a permissive license, companies could take it, modify it, and redistribute the modified version under a proprietary license, denying downstream users the freedoms the original code was released with. copyleft's "share-alike" requirement closes this loophole: anyone who distributes a modified version must distribute it under the same terms.
The ethical argument is that copyleft embodies a commitment to reciprocity. Free software is a community project sustained by mutual contribution; it is appropriate to require that those who benefit from it also contribute to it, at least by maintaining the freedom of any derivative work. This is not a restriction on freedom but a defense of it.
stallman also addresses the critique from permissive-license advocates — including many in the open-source-definition-schism camp — that copyleft is a restriction and therefore incompatible with the ideals of freedom. He argues that this confuses the freedom of individual developers (which copyleft restricts in one specific way) with the freedom of users (which copyleft protects). The goal is user freedom, and copyleft serves that goal even at some cost to developer freedom.
This essay is the briefest statement of the argument developed more fully in copyleft-pragmatic-idealism and reflected in the design of gpl-v2 as analyzed by eben-moglen and others associated with the software-freedom-law-center.