The commercial effect of a strong copyleft software license (e.g. the GNU General Public License [GPL] and lesser-known GNU Affero General Public License) varies drastically depending upon how the software is used.

For an end-user product, e.g. GIMP, strong copyleft is an elegant way to force companies to contribute their modifications back to the community.

But for a developer-oriented library or other building block, e.g. Neo4j Enterprise, strong copyleft effectively prevents companies from selling applications built using the product, and is therefore typically used in dual-licensing arrangements, as a way for the copyright holder to publish the source code but still sell commercial licenses.

Strong copyleft says it’s ok for a photographer to use GIMP in the pursuit of profit, but not ok for a developer to do likewise with Neo4j1. This is not an accident: it flows from the Free Software Foundation’s definition of freedom for software end users. Developers must be guarded against restricting this freedom while photographers can be left alone.

1. This is not entirely true for all strong-copyleft developer products, since some of them provide estoppel statements in the form of linking exceptions or permissive licenses for interface points. These statements exist so that certain types of works based on the product can be sold.