You just lost your job, through no fault of your own, or maybe because you did the right thing and blew the whistle on illegal and/or unethical behavior. You don't know how long it will be before you find a new job or how you are going to pay the bills until then. Your employer offers you some money to tie you over, and maybe some resources to tide you over until you get a new job, but you have to agree to a hundred pages of legalese. And you only have a few hours to decide, not enough time to have a lawyer look at it, even if you could afford to pay a lawyer. You are highly stressed, so even if you take the time to read it, you probably don't take it all in, and feel pressured to agree. And your employer also makes vague threats that about what will happen if you don't sign it, like having a hard time finding a new job or maybe having legal action taken against you.

Does that seem like a free informed decision?

Looking at it another way, anti-disparagement agreements are basically bribes to keep quiet even if disclosure would benefit the general population.