Operating Environment: essential industry background for open software licenses

Many copyleft licenses, including most of the Free Software Foundation’s GPL family, the Mozilla Public License, and the Eclipse Public License, require sharing alike only if you first share your work with others. Many companies that primarily build software services that they run for customers to use, without sharing any source code, have taken false comfort in the idea that they’ll never distribute copies of their programs with others, and used a lot of copyleft code. Depending on who you ask, allowing private changes and limiting copyleft to “distributed” code is either ethically imperative or a hindrance to effectively requiring others, especially software service developers, to share their work as intended.

Source: writing.kemitchell.com