When a company asks for your copyright
Companies that develop free software and release it under the GNU GPL sometimes distribute some copies of the code in other ways. If they distribute the exact same code under a different license to certain users that pay for this, typically permitting including the code in proprietary programs, we call it "selling exceptions". If they distribute some version of the code solely in a proprietary manner, we call that releasing a purely proprietary version of the program.
http://www.gnu.org/philosophy/selling-exceptions.html explains why selling exceptions is acceptable, though only barely. By contrast, releasing a purely proprietary version is outright wrong, like any other proprietary software.
Companies normally sell exceptions using code they themselves have developed. Since they hold the copyright on that code, they can legally distribute it in any manner, even in multiple manners in parallel. But what happens when you publish a modified version of that free program, and the company wants to include your changes in its version?
