BSD许可证
The BSD licenses permit forks to become proprietary software, and copyleft proponents say that commercial incentives thus make proprietisation almost inevitable. (Copyleft licenses can, however, be circumvented via dual-licensing with a proprietary grant in the form of a Contributor License Agreement.) Examples include macOS (based on the proprietary NeXTSTEP and the open source FreeBSD), Cedega and CrossOver (proprietary forks of Wine, though CrossOver tracks Wine and contributes considerably), EnterpriseDB (a fork of PostgreSQL, adding Oracle compatibility features), Supported PostgreSQL with their proprietary ESM storage system, and Netezza's proprietary highly scalable derivative of PostgreSQL. Some of these vendors contribute back changes to the community project, while some keep their changes as their own competitive advantages.