Public Domain status denotes more freedom than GPL. It allows all of the freedom of GPL and in addition, it allows the freedom of making proprietary modifications. Public domain gives person P the ability to make modified versions and give users no freedom in using them. The result is that people in general have less freedom. This might seem like a paradox: you give people "more freedom", but they end up with less. How can that be? It has to do with stretching the word "freedom" to include the ability to control other people. That kind of "freedom" tends to leave other people with less freedom. What happened with the X Window System illustrates this unambiguously (see http://www.gnu.org/philosophy/x.html). One way to avoid the paradox is to distinguish between freedom and power. Freedom is being able to decide your own activities and choices that affect mainly you. When someone can decide other people's activities, or choices that affect mainly others, that is power, not freedom. With this definition, the paradox goes away. Copyright is a power, not a freedom. Copyleft, by blocking this power, protects freedom. The GNU GPL guarantees basic freedom for all users, which otherwise they will tend to lose.