I would not have imagined even two years ago that expansion of copyleft would become such an issue of interest in software freedom licensing. Historically and for good reason, addition of new forms of copyleft clauses has moved at a steady pace. The early 2000s brought network services clauses (such as that in the Affero GPL), which hinged primarily on requiring provision of source to network-remote users. Affero GPL implemented this via copyright-controlled permission of modification. These licenses began as experiments, and were not approved by some license certification authorities until many years later.

“Most proprietary relicensing businesses work as follows: a single codebase is produced by a for-profit company, which retains 100% control over all copyright in the software (either via an ©AA or a CLA). That codebase is offered as a gratis product to the marketplace, and the company invests substantial resources in marketing the software to users looking for FOSS solutions. The marketing department then engages in captious and unprincipled copyleft enforcement actions in an effort to “convert” those FOSS users into paying customers for proprietary licensing for the same codebase. (Occasionally, the company also offers additional proprietary add-ons, improvements, or security updates that are not available under the FOSS license — when used this way, the model is often specifically called “Open Core”.)”