What if it raises it because now that a former employee is unencumbered by non-disparagement clauses, employers have an incentive to offer decent/generous severance to ensure the former employee is not a now disgruntled former employee?
If there weren't any bad things to talk about then people only got the minimum anyway.
This stops companies buying the silence of people who'd keep quiet for more cash, which stops bad companies who do bad things from hiding their bad things so easily and it stops people who'll keep quiet about bad things for personal profit from gaining.
That's a win win.