An employer's rule that employees who make complaints should not discuss those complaints with co-workers during an on-going investigation violates the NLRA according to the National Labor Relations Board. Specifically, the Board found that the employees' Section 7 rights outweighed the employer's generalized need for confidentiality during an investigation. Accordingly, the rule (which was arguably a practice) was found to violate the law. The Board also did not modify the ALJ's determination that the employer's written confidentiality agreement - which prohibited employees from discussing their co-workers' discipline or salaries unless disclosed by the co-worker - violated the NLRA. The decision can be found here.
The Board's determination in this matter is similar to their social media decisions which also struck down policies that contained broad prohibitions yet lacked specific caveats and/or and compelling needs for the policy. The Board's most recent memorandum on social media issues can be found here.