The key hack that made free software possible was a legal one: using copyright to keep software free. It did that by demanding a quid pro quo: if you use software made available under the GNU GPL, modify it and distribute it, you too must make it available under the GNU GPL.
If it were possible to take software released under the GPL, modify it and release it, but without passing on the freedoms to users downstream, the entire edifice of free software would be in trouble. And that, alas, iseems to be precisely what is happening in a German court case:
On Open Enterprise blog.