I agree on settlements not being legally required so they should still be able to be included in them. It'd have a huge chilling impact on people being willing to settle otherwise.
More specifically though, I think it's as I mentioned before, that the workers have statutorily mandated rights set out in the National Labor Relations Act. The judges are saying companies can't negotiate away these statutorily protected rights whether through severance payments or otherwise (otherwise what's the point of the statute if you can buy your way out of it is I think their argument). From what I've read in the articles (which as we agreed are not robust explanations so we might be missing huge parts of the picture), I don't think that it's that it's that's it's the fact it's severance specifically because:
1. (Google tells me) that no state or federal law requires paying severance. Severance is only required if it were a part of a contract with an employer - e.g. employment contract (rare to have unless higher level employee), union collective bargaining agreement, employee handbook, etc. Note that the union contracts I worked with didn't have severance guaranteed, so it's not even standard in all of them.
2. The Act is mentioned in the articles I read rather than focusing on the fact that it's severance.
From the article I linked above: "Today’s decision, in contrast, explains that simply offering employees a severance agreement that requires them to broadly give up their rights under Section 7 of the Act violates Section 8(a)(1) of the Act. The Board observed that the employer’s offer is itself an attempt to deter employees from exercising their statutory rights, at a time when employees may feel they must give up their rights in order to get the benefits provided in the agreement."
Section 8(a) states: "It shall be an unfair labor practice for an employer--
(1) to interfere with, restrain, or coerce employees in the exercise of the rights guaranteed in section 7 [section 157 of this title];..."
Section 7 states: RIGHTS OF EMPLOYEES
"Employees shall have the right to self-organization, to form, join, or assist labor organizations, to bargain collectively through representatives of their own choosing, and to engage in other concerted activities for the purpose of collective bargaining or other mutual aid or protection, and shall also have the right to refrain from any or all of such activities except to the extent that such right may be affected by an agreement requiring membership in a labor organization as a condition of employment as authorized in section 8(a)(3) [section 158(a)(3) of this title]."
I really should find a copy of the decision. Now I'm curious.
Disclaimer: Even though I've worked with unions, I am not an employment/labor attorney, I am not your attorney, nor am I providing legal advice.