I got hit in the layoffs at Halliburton here in Houston back in January, and at no time was "don't disparage the company" ever made a condition of accepting severance. In fact, the only thing I had to sign said "I've returned all of the company property I might have", and that was it.
Good point. At large companies, sometimes they don't include it for non-executives. At smaller companies, it should probably fall on everybody since each voice carries more weight and everybody would have more insight into the actual workings of the company.
It's all about making a legal guard against employees trying to "retaliate" over being suddenly released from employment against their will. At large employers, edge nodes in the employment graph can't do much harm to the overall company because they probably don't have insight into the deeper machinations of what's really happening outside of their compartmentalized roles (employer->employee power dynamics are fun).
