The US law in question deals with the right of employees to organize and protect their interests (e.g. unsafe working conditions, unfair pay practices, et-al).
Towards this end they must be allowed to communicate. They may form unions if management is not willing to participate in those discussions, but that isn't a mandate of the code. Thus the limitations placed on their ability to communicate violate the law, not the use of company resources towards that end. It would be just as illegal if the company attempted to limit negative commentary using pencil and paper versus bits.
If DISH network had said something like You can't send nude pictures via our e-mail system the ruling would have been in their favor. Carte Blanche you can't criticize us electronically impacts the employees ability to discuss working conditions.
On a side note many US companies have policies saying you can't discuss salary with other employees. That too violates code with regards to unfair pay practices (e.g. gender discrimination)
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