Of course, this is really only a significant issue on a public forum, on which you can post pseudonymously and still gain any benefits of discussion without paying any significant toll. Some people view this as a negative and want to ban this kind of commenting - I love it, it means you can experiment with different views, make arguments you might not want to make in real life and see what sticks with you.
You can say that about almost anything. I'm at liberty to murder my neighbor, I just have to accept the price.
Denying your neighbor the ability to speak out against your opinions violates their right to free speech. If you don't like it, argue back.
the quoted 'at liberty' seems to refer to something like not being restricted by the law of the land.
saying 'at liberty to murder' seems to refer to something like bodily autonomy/physical capability.