There are limits on what is enforcable in a contract. Like, you cannot enslave yourself. It is true that that limits freedoms in some sense. But it serves to protect freedoms that society at large deems more important.
Sure, but (a) RMS actually does make that argument and (b) he AFAIK doesn't actually call for making proprietary software illegal, but just tries to convince people to not produce, endorse, or use it.
