Yep, GPL preserves freedom in exactly the same way modern societies preserve freedom for their citizens. You are free, everyone who lives in the country is also free and you're not allowed to enslave other people. Of course, someone could make the argument that preventing you from owning slaves denies you some freedom. And from a narrow and purely technical side he would be right. However, which society would you say has more freedom, the one in which slavery is forbidden, or the other? I think the same argument applies to FOSS.