WordPress is distributed under the GNU/GPL license. You can open source code of any pligin or theme and see there “GNU/GPL”.
“The GNU General Public License (GNU GPL or GPL) is a widely used free software license, which guarantees end users the freedom to run, study, share and modify the software. The license was originally written by Richard Stallman of the Free Software Foundation (FSF) for the GNU Project, and grants the recipients of a computer program the rights of the Free Software Definition. The GPL is a copyleft license, which means that derivative work can only be distributed under the same license terms. This is in distinction to permissive free software licenses, of which the BSD licenses and the MIT License are widely used examples. GPL was the first copyleft license for general use.”
GPL license provides web-designers the following rights or “freedoms”:
- The freedom for starting programs with any purpose.
- The freedom for exploring and researching of how that program works. That means we can open source code.
- The freedom for distributing copies of sourse code and executable code.
- The freedom for improving program and publish it on the Internet.
That means that we jointly and absolutely legally purchase the access to source code (files) using club subscriptions. After that you are free to do anything with source code.