The difference can be broadly characterized as individual vs. societal freedom. To resolve your problem with the meaning of "free", you have to think about who

The difference can be broadly characterized as individual vs. societal freedom. To resolve your problem with the meaning of "free", you have to think about who the intended recipients of the freedom are. If there are no restrictions against (to pick a colorful example) letting your dog go to the toilet on the sidewalk, then individually you have more "freedom to do whatever I want", but societies work better when you can walk around without constantly stepping in dog poop. So, the way I look at it is that the GPL attempts to build a society where total happiness is greater than in the society that gave more permissions to everyone. If you agree with the existence of laws that prevent poor behaviors, you're already sympathetic to this view in principle; it just becomes a practical question of whether the harms that the GPL is trying to avoid are serious enough for it to make it worth the trade-off with individual liberty. Reasonable people disagree about this.