macstl is dual-licensed under the open-source Reciprocal Public License (RPL) and the proprietary Pixelglow Source License (PSL).
The way it works is this: you either reciprocate your own code, or pay for a proprietary registration. While it is an open-source license, the RPL prohibits you from keeping your own derivatives proprietary even if you only deploy in-house. On the other hand, the paid registrations also come with priority support, version control access and free upgrades for differing periods.
Created by the fine folk at Technical Pursuit, the RPL has been certified by the Open Source Initiative as open source since January 2003.
This scheme tries to balance the ideals of open source and the need to make a living from software.