[xvdAZh]

Just a layman, but the reasoning given by the NLRB kind of makes sense to me. FTA:

> Public statements by employees about the workplace are central to the exercise of employee rights under the Act.… [T]he comprehensive ban would encompass employee conduct regarding any labor issue, dispute, or term and condition of employment of the Respondent.… [E]mployee critique of employer policy pursuant to the clear right under the Act to publicize labor disputes is subject only to the requirement that employees' communications not be so "disloyal, reckless or maliciously untrue as to lose the Act's protection.

In other words, I think it's because these discussions could be said to be part of collective bargaining: the text of these sorts of agreements is something that, at the very least, your union (or coworkers who're interested in forming one) would want to know about, right?

My (uninformed) guess is that the Court of Appeals would rule along the lines of "the NLRB can regulate away banning these clauses from applying to discussions with coworkers and union representatives, but not the general public," since banning them in general could be considered as acting outside their regulatory mandate.