The PublishPress plugins are free software; you can redistribute and/or modify them under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.
All existing copyrights must be prominently visible on all modified versions of this software.
The products are distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. Please read the complete GNU General Public License for more details.
PublishPress does use a license key system, but that is only for automatic updates and support. There are no extra restrictions on the codebase, beyond those implemented by the GNU General Public License.